Then you don't understand what being designated as Commander of an aircraft actually means.
Actually I do.
Yes there can be only one commander at any one time. However I suspect the more senior might find themselves culpable if they did not assume control when faced with a breach of the law.
Rubbish. The commander is the commander full stop. Any passenger is just a passenger pure and simple. Christ, I would never be able to fly as a passenger with another pilot for fear of being prosecuted for their mistakes. When I am teaching I am responsible, when I am a passenger I am there to look out the window.
Reminds me of the Instructor who tried to convince everyone he flew with that he must be P1 because he is an Instructor. God complex......