parabellum, if you go back through these posts you will see that several Captains have stated that as PIC they are in command and have responsibility.
Now I don't know if in this United case that is true or not but if the PIC is in charge from say time of aircraft entry then your comment of layers of management are not correct.
As many of the posters on this thread now see, there seems to be a void in when responsibility or as said "the can" is handed over.
I personally don't think this Republic Airlines Captain had a clue on what was about to happen or what did happen till well after the event.
From what I have read from FAA I also don't think he can be responsible for being PIC.
But he maybe accountable for the end result if he made or approved the call to bring on board the offending thugs.(That is a maybe).
I will state I have no idea of the procedures or ops manuals for United or Republic Airlines or contracts they have between each other, and in fact this may well be where the final responsibility is known - but I doubt it.