"With all crew and passengers incapacitated from smoke inhalation, how is it possible for a plane, on fire, to continue flying for some 7 hours?"
You have assumed that it kept burning for 7 hours. You also assume fire took out all communication systems. Protocal dictates that you switch off circuits one by one until you find the faulty one. The procedure for a fire is a long checklist and needs examining. Oxygen was limited, it's likely that the fire used up all available oxygen then went out by itself. Engines, flaps etc will stay doing what they were doing. This possibly means the plane flew level until fuel ran out, then glided for hours after that
