There are several examples of “ghost planes” where the flight crew is unconscious due to the cabin depressurizing. The flight then continues on autopilot until the fuel is exhausted. By the way, one of the initial side effects of hypoxia is euphoria. You can find examples on YouTube and there is a great video by 74gear demonstrating this - see https://youtu.be/nz5d4Q_ykFc.

Depressurizing the cabin to incapacitate the other crew is definitely a possibility and I think the leading theory for what happened. Like you say there is no proof either way - so we may never know with 100% certainty.