4. If this was a case of cabin depressurization, and resulting hypoxia, then the aircraft would not have been able to be turned around to another flight path?
Is it possible that the crew were partially incapacitated from insidious hypoxia caused by a slow decompression and tried to reprogramme the FMS to take them back to Kuala Lumpur. However, they failed to do this correctly and the aircraft turned onto an approximately southerly track, taking them past but not to Kuala Lumpur?
With complete unconsciousness and death following some time later for everyone on board, the aircraft just flew on this southerly track until it's fuel ran out, shortly after it gave its last ping, at which point it was positioned on the southern arc.
In other words, could this still possibly be just a terrible accident? No suicides, no hijackers, no nefarious goings on of any kind?