Like the decompression theory, burning Li Ion batteries in the hold cannot explain the aircraft logging off ACARS (not just dropping out) then ten minutes later switching of the transponders, then turning West and flying for another few hours. (Assuming all that information is correct)
Actually, an electrical fire would explain all of those things. For one, if there is an electrical fire (or a suspect electrical fire) turning off power to the affected system is exactly the right thing to do. It also could explain the flight path deviations because the an electrical fire in the cabin would not affect the engines but could affect the ability to control the plane.
FAA: Some Boeing 777s need fixes in case of fires
Batteries are NOT the only thing that can cause an electrical fire. The in- flight entertainment system can cause them too, see article above.