He was not trespassing. He paid for the seat he was sitting in. He was allowed to board the airplane and was not in violation of UAL's Contract of Carriage.
He did not interfere with the flight crew. He merely stated he was not going to deplane because the flight crew had no rights under the CoC to remove him from the flight. (See: RULE 21 REFUSAL OF TRANSPORT)
The section on RULE 25 DENIED BOARDING COMPENSATION does not apply because that section deals with denying boarding to a passenger due to oversold conditions.
A) the flight was not oversold.
B) He was allowed to board and was already in his seat.
...failure to obey a lawful order.
What lawful order was given?
Once he was identified by United that he was no longer going to fly on the aircraft, he was trespassing. Once he was trespassing and refusing to leave, he was interfering with the operation of the aircraft. The lawful order was to vacate the aircraft.
1) yes the flight was oversold. It became oversold when more passengers need seats than are available.
2) his being allowed to board is irrelevant