You guys can argue "legalities" of who is responsible at whatever point as long as you like.
The reality is the aircraft goes nowhere till the Captain is happy.
If some dip**** is blueing over seating they will be removed before flight, one way or another.
( not because they are wrong with being upset, because they are blueing about it! )
If the United Captain had made a PA that due to a seating problem EVERYONE is to get off, and then the good DR had not been given a new boarding pass then the issue would not have happened?!