Water closet derivation is given above. Accepted john derivation is given below.
Sanitary Ramblings - https://archive.org/details/b22014834/mode/2up
Brits were big on euphemisms since they hesitated to say things like 'toilet' and 'excrement'.
The water-closet with flush toilet was designed to allow one to achieve the Victorian ideal of faecal denial. The loo and the water-closet concealed the sight, sounds, and smells of poop, making them everyone’s dirty little secret that no one, not even the servants, needed to be privy to (pun intended) any longer. Consequently, contemporary culture became a culture of faecal confusion. Everyone knew that everyone pooped, but everyone of culture pooped using apparatus and facilities specifically designed to create the appearance that no one did.
According to the OED, a water-closet is “a closet or small room fitted up to serve as a privy, and furnished with water supply to flush the pan and discharge its contents into a waste-pipe below. Often abbreviated W.C. or WC''. The term has been in use since 1755.
The water-closet was the room with the toilet, whereas the bathroom was the room with the bathtub. Water-closets date from the mid-1700’s but didn’t become common until the mid-1800’s. By the late 1850’s it was very fashionable in England to have an indoor water-closet, and indoor plumbing became a status symbol. The WC was often located off the landing halfway up the stairs from the first to the second story (hence the alternate term “halfway house" for lavatory).
By 1870, the gilded throne of feudal monarchy had given way to the porcelain throne of Victorian morality. The Victorian demand for faecal denial had finally engendered an infrastructure that could truly provide it. Now the affluent could emerge from their water-closets with no evidence whatsoever left behind, as though they didn’t ever poop at all. The lower classes, enveloped by the stench of their privies, close-stools, and chamber pots, could only stare in envy and dream of the flush toilets they could never afford. It would be decades before they could.
‘Closet’ has become a universal all-encompassing term for ‘toilet’ in Britain, used even in applications far removed from a water- or earth-closet. For example, ‘closet’ is frequently used interchangeably with ‘privy’, such that terms like ‘midden privy’ become ‘midden closet’.
In 1735, there was reference made to a toilet enclosure, using the term ‘cuzjohn’, an abbreviation of “cousin John”, as in “going to see cousin John”. The same year, this term became just the word ‘john’, meaning ‘the place of easement’. By the 1800’s there were several different parallel proper names in use: the Joe, the Jane, the Fred, Miss White’s, and the Widow Jones, to name a few. Of interest to ripperologists: ‘john’ also, of course, means a prostitute’s client (since at least 1906) but during the 1800’s, ‘john’ alternatively meant ‘policeman’ (as in John Law), derived from an abbreviation for "John Darm," a pun on the French ‘gendarme’.
Following are some of the words and phrases used in Britain to mean ‘lavatory’: bog, cloakroom, close stool, closet, commode, convenience, garderobe, gents, heads, khazi, ladies, latrine, loo, necessary, netty, place of easement, powder room, privy, shithouse, smallest room, thunder-box, toilet, water-closet, and WC.
For ‘go to the lavatory’, these euphemisms are commonly used: ‘explore the geography of the house’, ‘go to the restroom/cloakroom/loo/toilet’, ‘pay a visit’, ‘powder my nose’, ‘visit the smallest room’, or ‘wash my hands’. To say ‘Please may I be excused?’ usually meant to request permission to go to the lavatory.
‘Privy’ used to be a general word for `lavatory' or `toilet' but was used especially for a shed or hut, separate from the house, which contained seats over a cesspit or privy vault, an earth closet, or sometimes a water-closet. Modern dictionaries say privy is an American word for an outside lavatory, but according to the OED, the word privy is 600 years old, and means “a private place of ease, a latrine, a necessary”; hence the derivative terms privy house, privy stool, and privy member. A privy was called by various names, such as `netty' in the northeast of England and `cludgie' in Scotland. Other names associated with ‘privy’ (and frequently, ‘toilet’) are loo, thunderbox, shitter, shithouse, back house, outhouse (in USA), water-closet, outdoor convenience, chamber of commerce, holy of holies, cloakroom, shot-tower, ‘smallest room’, ‘john’ (dating from 1735) and ‘place of easement’.

New topic:Early toilets were cast-iron contraptions that flushed inadequately (leaving ‘skid mark’ smears and clogs) and frequently gave off an awful stench because the water trap had not then been invented. Even with the water trap, many people thought it terribly unsanitary for the privy to essentially be brought indoors. They warned about the dangers of sewer gas, claiming that it caused sickness and death due to ‘miasma’. It was because of these fears that the toilet first began to be separated from the bathroom and placed in its own room, the water-closet.

What's Old is New:
So far as the common multitudes were concerned, bodily functions were merely something to be performed as the need arose, anywhere and anytime, by just about everyone, and their surprising lack of discretion in doing so caused a vexed British Royal Court to issue this warning in 1589:
"Let no one, whoever he may be, before,at, or after meals, early or late, foul thestaircases, corridors; or closets with urineor other filth."
Etiquette books of the day instructed that it was impolite to greet someone who was in the process of urinating or defecating. If one were to observe someone in the act of relieving himself or herself, advice was given that one should act as if one had not seen them.
San Francisco? No, Olde London.

The River Thames, for the millions of Londoners who drank from it, was as much liquid biology as it was liquid history. As written in 1834, “he who drinks a tumbler of London water has literally in his stomach more animated beings than there are men, women and children on the face of the Globe”. See Figure 76. At least 140 sewers discharged their contents (which contained an ever-increasing proportion of household sewage) into the Thames in 1828, and the growing use of water-closets in the 1830’s and 1840’s meant that the problem had become much worse by the middle of the 19th century.
The streets of London lie 30 feet below the surface of the Thames at high tide. Until Joseph Bazalgette completed his banks of pumped intercepting sewers circa 1866, this meant that at high tide, the Thames would flood many of the sewers emptying into it, causing raw sewage to backflow and spill out at the lower elevations. Even worse, it meant that contaminated river water and raw sewage would be backed into some of the intakes used for pumping drinking water. Epidemics of cholera, typhoid, dysentery, and other ‘diseases of sanitation’ thus plagued the City on a regular basis. Social reformers of the era struggled with upper class apathy toward the horrible sanitary conditions in the East End. Edwin Chadwick attacked the greed and indifference of property owners, stating: "Early in the progress of these investigations, the proposed system of cleansing, by removal of the ordure in suspension in waste, was objected to on the grounds of supposed loss of money received for manure." The construction of large central covered sewers had already begun in 1844, even though no plan had then been drawn to replace cesspool and cesspits.
Impure or contaminated water supplies had always been a problem wherever humans congregated and lived en masse. Romans routinely mixed wine with their drinking water, having observed that alcohol-infused water prevented or mitigated diarrhea and other intestinal ailments. This habit, brought to the British isles with the Roman occupation, may in part explain the traditional British cultural propensity toward drink (think Oliver Reed), which persists to this day. And why not? - dying of cirrhosis of the liver in your forties was a lot better than dying of dysentery in your twenties. Some fortunate few escaped the cholera epidemic in 1854 Soho by inadvertently drinking contaminated Broad Street well water that had been mixed with brandy. Others escaped by habitually drinking nothing but malt liquor, in which pathogens such as V. cholerae cannot survive.The Ghost Map, Steven Johnson, Riverhead Books, 2006, page 103
Even with the excellent health hint left by the Romans, health authorities of the day ignored the ramifications of purifying sewage-contaminated drinking water. This should really come as no surprise, as mortality rates had been observed to go down significantly once tea drinking had become popular. Boil the water, drink the tea, and your chances of surviving cholera, typhoid, and other ‘diseases of sanitation’ improved immensely. But it all went for naught due to the health authorities’ obsession with the then-prevailing disease-transmission theory of ‘miasma’.
I sent you a PM regarding this article.

New!: https://www.atlasobscura.com/articles/zombie-river-thames

That is still true - the river did just stink but it presented no danger from combustion or explosion. In the enclosed volume of a privy, however, flammable gases could concentrate enough to present such a danger - and often did. I went deep-sea fishing in 1967 and there was a big sign on the head (lavatory) door - NO SMOKING. A week to the day later, some tourist ignored the sign, smoked in the head, threw the butt in the toilet, and the resulting fire caused loss of the boat - The Marlin Queen.
This article explores the means and methods for disposal of human excrement throughout British history, ending at the time of the Whitechapel Murders with an inside and in-depth look at the humble privy. It is intended to educate and provoke thought, not to titillate or disgust, though, unfortunately, this subject matter has for millennia been twisted and perverted into no end of crude scatological jokes and indecent references. Long considered an unacceptable or taboo discussion topic, socially and academically, the sanitary habits of our ancestors have not been extensively documented; those academics who did dare to research and write on the subject were once and not so long ago considered vulgar, lowbrow, and in some cases even erotic. Even as late as 1962, Jack Paar, the original host of ‘The Tonight Show’, was censored by the NBC television network for merely saying ‘WC’ (abbreviation for water-closet) on a live broadcast. With persecution like this for media missteps of so minor a nature, contemporary Victorian authors such as Charles Dickens and Conan Doyle weren’t about to jeopardize their publishing contracts and literary careers by mentioning privies or anything else scatological (although Doyle did use the word ‘cesspool’ in 1887’s A Study in Scarlet and the word ‘bathroom’ in the subsequent The Man With the Twisted Lip). It was a social quirk of the Victorian Era that forthright expression or description of bodily functions and other scatological matters was expressly forbidden, even though, as we shall see, urban life during much of this period literally revolved around avoiding human and animal excrement.
But all that was then and this is now. Today, we find in mainstream media outlets, such as Amazon.com, children’s book titles like Everyone Poops, Where's the Poop?, and The Truth About Poop, all of which expose youngsters to graphic details of a common bodily function that their parents, and certainly their ancestors, had diligently avoided during polite conversation. For adults, there are now available titles like A Sociological History of Excretory Experience: Defecatory Manners and Toiletry Technologies and Merde: Excursions in Scientific, Cultural, and Socio-Historical Coprology. Heavy reading here, to be sure, and probably a good deal more cerebral than the plain and unadorned History of Shit, also available from the same sources. Believe it or not, there are even a number of websites expressly dedicated to all conceivable things scatological. For those who dare, try a visit to www.poopreport.com, www.heptune.com, and www.doodie.com. Beware of the latter if you are easily offended.

Okay, Versailles I have not covered. Maybe in the sequel.......

Our language today is full of euphemisms to describe such activities, e.g. "waste management". We have the "restroom", the "washroom" and the "bathroom" as though we were going for a rest or a wash or a bath when we excuse ourselves. Of course no one actually wants to rest in the room containing the toilet; restroom is an obvious euphemism. Interestingly, English can express the "toilet-room" concept only via indirect terms like this. The French are much more straightforward – ‘pissoir’ means “place to piss” – but English has no similarly unadorned word aside from ‘shithouse’, which is not exactly usable in polite conversation anywhere outside of metropolitan Philadelphia. A privy might be called something other than the “outhouse”, the “John”, the “Necessary”, the “Back House”, the “Friendly”, the “Closet”, the “Throne Room”, the “Reading Room” or the “Library”. You might announce to the world at large that you are “going to the woodpile”. You could go off to “see a man about a dog”. A lady might “go to see Aunt Mabel” or any other fictitious “aunt”, or “powder her nose”, an act that has been described as “a private errand involving neither powder nor nose”. A “man in a hurry” was a man who was in need of a water-closet, privy, or equivalent facility – and fast.
So far as the origin of the vulgar slang term “crap” and its variations is concerned, it is still much debated. Etymologists declare that the name of Sir Thomas Crapper is not the origin of the word, instead maintaining that it is derivative from either the Middle English “crappe”, meaning a scraping or the residue from rendered fat, the Old French “crappe”, meaning chaff or residue, or from the Medieval Latin “crappa”, meaning residue. Yet another possibility is the German word "krappe", meaning a vile and inedible fish, not to be confused with the ‘crappie’, which is not at all vile and is quite edible.
Privy vaults also generated methane gas from the decaying organic waste they contained, and this gas, being lighter than air, would rise up through the privy vault’s only exit, the privy seat, and accumulate within the privy structure. In an age when tobacco was ubiquitous, the consequence from a user striking a match to light his pipe or cigar while ‘on the seat’ could be explosion, fire, and not infrequently, death. Fires and explosions, with resulting deaths, were common in London’s sewer system during the period when the city’s scavengers entered them bearing oil lanterns. For these same reasons, smoking is forbidden in ships’ heads today.

The classic scatological term ‘shit’ derives from the Olde English noun ‘scite’ and the Oulde German ‘schite’, both terms meaning "dung". Its use as both a noun and a verb has been traced in written works as far back as the 14thcentury. Its synonyms are too numerous to list, this condition being complicated by the fact that the word is used as a noun, verb, exclamation, adjective, and numerous other parts of speech. Many such terms are colloquialisms, but the most common today are poop, doo (also doo-doo or doody), poo, and crap. ‘Night-soil’, ‘dust’, and ‘filth’ were the most common Victorian euphemisms. For some reason, ‘humanure’ never caught on.
While we’re on the subject, or near it, anyway, common euphemisms for ‘urinate’ are: Jimmy Riddle, micturate, pee, piddle, piss, pump ship, slash, strain the potatoes, spend a penny, water the garden/tulips/tomatoes/mule, wee, tiddle, and, for men, ’point Percy at the porcelain’ (my personal favourite; it sounds so Victorian), or ‘shake hands with an old friend’. Those in Tudor England would happily ‘pluck a rose’ (have a wee) anywhere – in chimneys, corners of rooms, stairways, or in the street. It was these kinds of indiscretions that led to that Royal Warning issued in 1589.

Well, that article is 87 pages long.

A ‘honey bucket’ is a bucket that is used in place of a flush toilet in areas that lack a sanitary sewage system. It is the same as a pail privy pail. The ‘honey bucket’ gets its name from the five–gallon buckets used for this purpose, which were the same type once used by beekeepers as containers for honey. See Figures 13 and 14. Accordingly, the nightman who serviced the ‘honey-bucket’ began to be referred to as a ‘honey-dipper’ and he drove a ‘honey wagon’. Such terms made for much nicer euphemisms. Their etymology is not precisely known, but the terms were in use prior to 1901. Even today, vacuum trucks that service septic tanks and local collections of sewage, such as at construction sites, are termed ‘honey wagons’.

It was the task of the nightmen to excavate and haul off the ‘night-soil’ deposited in cesspools, cesspits, and privy vaults, labor referred to as "nightwork" because by law it could only be performed between midnight and five a.m.The work of emptying cesspools was "sometimes severe"; work conditions were foul and could be deadly, and there were numerous fatalities over the centuries, caused by a raker, gong-fermor, or nightman falling into a deep cesspool during the cleaning process and literally drowning in human excrement.
By the 19th century, nightmen had developed a precise choreography for their labors. Nightmen would arrive on site in teams of three or four with a cart. One man, the ‘holeman’ initially went to the cesspool and filled a tub either by immersing it directly into the night-soil or by means of a shovel. Eventually, as more and more night soil was removed, the nightmen would lower a ladder down and the holeman would descend into the pit and scoop waste into the tub. Then he would scrape or wash off the outside of the tub before the ‘rope man’ would raise the tub out of the pit, and the two ‘tubmen’ would carry the filled tub, suspended on a pole between them, to a cart for emptying. See Figure 21. Figures 22 and 23 illustrate tipping carts of the type used by nightmen in the 19th century for bulk removal of night-soil. These carts were fitted with hinged covers to provide easy access, to facilitate disposal, and to suppress the sight and smell of their repugnant contents.

I'm not sure it is the worst, but it is right near the top if not. Ah, the smell of decomposing flesh....
Sorry to hear about your mom's experience, but it could have been worse:In 1184, when Emperor Frederick of the Holy Roman Empire held a Diet in the Palace of Erfurt, the floor of the main hall collapsed. Many of the dinner guests fell into the cesspit directly beneath and drowned; luckily, the Emperor survived. One of his companions, who perished, had been in the habit of swearing, "If I do it not, may I sink in a privy." Close enough, I reckon.
“There is one privy, which has a cesspool in common with another privy attached to another house. The cesspool is nearly full; the woodwork of the privy can scarcely hold together, and is dangerous to use. Not long ago the landlady of some houses fell into a cesspool and was suffocated.” (Bethnal Green, 1848)
Death By Privy was not all that uncommon. When a privy was shared among multiple households, it was usually located atop a privy vault, which, like cesspools of the time, might be up to 20 feet deep to provide adequate holding capacity between cleanings. These privies were generally of wood or brick construction, but the flooring was usually of wood. Wooden privy floors (as well as floors above cesspits) had a disconcerting tendency to fail catastrophically, due to the corrosive effect of the wet hydrogen sulfide gas liberated by decomposing sewage. if a wooden floor thus weakened by corrosion were to fail by dint of someone’s weight, that someone would then fall into the privy vault (or cesspit), with lamentable results. It was also not at all unusual for someone to fall through missing or unsecured floorboards in a privy at night and drown or suffocate in the pit of excrement below. Even if the incident were not immediately fatal, the victim would still be in deep shit.