Second - this was a case of a passenger disobeying a lawful instruction from aircrew after boarding
We need go no further, the instruction came from the ground staff, not aircrew, who should have known that attempting to offload a well behaved passenger who has been accepted for the flight, issued a boarding pass and has boarded the aircraft, simply because you want his seat to accommodate a positioning crew, is not a lawful instruction from anybody, it is a request which the Doctor declined. The problem is an internal one for the airline to solve. Quite why a member of the positioning crew wasn't told, for 45 a minute flight, "It is the jump seat or nothing, exigencies of the company override" I doubt we will ever know.
In all probability the lawyers for the Doctor won't even bother themselves with the niceties of ticketing, boarding, offloading etc. they have all they need to pursue GBH, serious injury, humiliation, loss of earnings etc. a video, a hospital report and the statement of the CEO UAL.