So, with all of the theories abound, I'm with you...some sort of catastrophic electrical failure followed by sort of incapacitation. If it was a hijacking, that was a failure - because usually they want to make some sort of statement with it.
But, a couple of questions: if they had an electrical malfunction severe enough to take down all communication options AND cause them to descend to clear smoke, would it have affected other fly-by-wire control systems or autopilot? How separate are those systems? You've also got other people on the plane - flight attendants, etc. It would have to be pretty bad to get everybody. What are the chances of an electrical failure that caused a couple of issues including a depressurization that might have gone somewhat unnoticed - maybe that electrical failure affected the instruments that indicated the problem? I know you should be able to recognize that but we had that private jet several years ago with a faulty door seal that knocked everyone out.
