The media is reporting that the last heard radio voice comm sounded more formal than previously reported. Without going into the reasons for this (astonishing) change of story, does it not change the possible scenarios? That the co-pilot was fully awake and that he had been handed off to the next controller. The first controller would assume him to be off frequency and the new controller has not yet had first contact. The ideal time to do a vanishing act. So it says to this Poirot that the captain was not there or incapacitated and the FO was the instigator.