Sometimes when I can't make up my mind about a case or when there are too many possibles, I am reduced to ruling oout the possibles and seeing what is left.
Issues with plane: the plane flew for 7 hours....... no pilot or crew was able to make radio contact with anyone? If the plane was on fire, pilots would have made radio contact. If problem with nagivation, pilot would have made radio contact and would have shut off auto and flown manually. If problem with communication, plane would have headed for airport.
Pilot issues: the plane went too low, it went too high, it took a sharp left. Habit as well as pride would have made a real pilot flying as smoothly as possible. No up and down burning fuel, no attention getting moves at all. They would most likely have tried to make occasional radio contact to make things appear normal or to give some explanation for their moves.
The only other possible I see is a hijacking. Inexperienced pilot, someone who knows the procedures for flying but doesn't have the smoothness that comes with experience.