Be good to hear speculation from commercial pilots. It looks like the transponder stopped sending around the time the plane changed course. However, the flight didn’t lose much altitude over the few hundred kilometers that it travelled after turning around.
Is a complete electrical failure plausible? No transponder, no radio, no navigation, but still engine power. Could the pilots have been left with trying to visually navigate back to Kuala Lumpur and missed their destination?
If they had a complete electrical failure, it is gone down. The 777 is fly by wire. Now losing nav and radios they should have flown a holding pattern at max conserve airspeed and attempted the recovery in daylight. Looking at the cockpit I don't see a wet compass or a spirit-based turn and slip indicator but that doesn't mean there isn't one. But that is quite a reach back knowledge wise for a guy with 18K hours. Having to do partial panel holding for 6 or seven hours.