From reading your posts I'm figuring you are Ex Air force and know ALOT about large jet aircraft. Mountains more than me.
It looks like to me there was a mutiny of the flight crew or a hijacking of the plane. It's apparent that the aircraft flew another 4 hours after the transponders quit sending signals. With your experience where would you take it?
I’d hope they made an ~1200 mile circle with the center at last official known position, and then again at last “ping” position (smaller diameter) and then have some type of software run a visual search for clear areas on land masses between 7500 and 10000 feet in length with little or no vertical relief...
You keep the bird in the air first, then figure out where you are, the last thing you do is let others know your plight.
I had a crippled bird that had an engine explosion on rotation. The flight crew got off a Mayday once and we never talked to them again until the jet was on the runway.
In any aviation accident you have these factors:
-Pilots screwed up
-Maintenance screwed up
-Aircraft screwed up
-Outside forces (hijacking, bomb, lightning, birds, hail, ice, wind shear, volcanic ash, etc)
It is entirely possible they may have had a catastrophic failure that killed all comms. The westward turn would be consistent with them trying to get to land (no one wants a water landing)if the flight deck was dark (no displays) they may not have known where they were and maybe was hoping for visual references.
Aircraft have redundant systems but sometimes the boxes are right next to each other in the electrical bays so a fire or explosion would tend to kill the redundancy feature.