Surely, in the history of modern aviation (i am talking the jet age using vacuum loos), would this event not have replicated many times? Not just on aircraft, but anywhere human waste is collected? That theory is a non starter imho.
Regarding the bomb theory. If there is a rogue baggage handler, cabin cleaner, ground ops person etc. at Sharm (or where the aircraft originated), would you not keep quiet? The opportunity still exists to do it again? The fact that it is a Russian airliner also adds weight to that theory, in that Russia have only recently started to fight IS in Syria.
I think we are down to 2 theories. A fuel tank explosion, which rare, is not unprecendeted in modern jets, or a bomb, either planted prior or carried onboard. Remember, the passengers were not 100% Russian.....
Both theories would lend themselves to the flash seen by the US satellite. The tail coming off (again imho) would not have caused a flash.