cessnapuppy, I'll put aside a tangential romp in the realm of American jurisprudence.
cityfan, who appears to have great knowledge of the whole affair from the captain's perspective, categorizes United's investigation as a "flight safety" investigation.
cityfan also states that if it had been him, he might have had the purser arrested in Miami for interference with the crew. And cityfan also posits that after the diversion to Miami was made known, the purser "DEFINITELY DID interfere with the crew".
So, IMO, from the captain's standpoint, the situation on board had deteriorated to the extent that, in his judgment, flight safety was being compromised or potentially compromised, and a diversion to Miami was necessary and appropriate. That is the crux of the issue, and presumably the focus of United's investigation. There is little disagreeing that the captain had the authority to do what he did, and the issue seems to be whether he had a well-founded basis (flight safety) for exercising that authority.