So the airplane was aloft for 6 hours and 30 minutes. Which is just about the same (slightly less) than the planned flight with their reserves.
So a simple person like me thinks, pilot incapacitation, (rapid decompression) (think quantas flight 30 oxygen bottle,) enough time and wherewithal to gain control of the airplane, re engage the autopilot and engage heading select and lose consciousness ( think helios flight 522) . Airplane drones on until running out of fuel and the rest is history.
If its a hijacking, obviously they do not have a clue after taking control
Departed: 00:41AM
Last confirmed communication between the plane and the satellite: 8:11AM