As for #5, it's actually quite impossible for a nuclear reaction to occur in any implosion device by setting off the explosives through fire or other means. The core in an implosion device requires perfectly timed explosions happening at equal magnitudes from all angles all beginning at as close to simultaneously as possible. An explosion by fire or excess acceleration would detonate the conventional explosives asymmetrically, so it would have ZERO chance of going supercritical. There has been a lot of incidents involving nuclear weapons that have been involved in aircraft accidents that did nothing but cause their conventional explosives to go off. One happened in Palomares, Spain, where 4 hydrogen bombs were dropped after a midair collision between a B52 and a fueling aircraft. I believe two the conventional explosives in two of the bombs detonated and spilled radioactive materials, but there was no nuclear explosion. If anything, the reliability of nuclear weapons and their radioactive materials as well as relative volatile conventional explosives are much bigger problems than accidental detonation.