Unpleasant situation, no doubt.

I wonder what the protocol for the crew is. I imagine there is one. If a passenger has a medical issue (even if it is a mental health problem, such as a panic attack) and states that he wishes to leave the aircraft, I suppose the crew have to comply if it is doable?

I experienced these kinds of delays several times while taxiing. Each time the delay was about 1,5 hrs. Once, with AF on a CDG-EVN flight, a passenger who was about to be deported from France started to act up and the plane returned to gate immediately to offload him. Once he was offloaded, the captain came on the intercom to apologise and explain that it was company policy to offload anyone who was acting up on the ground, no matter what. He also explained that the lawyers for the deportees know this and instruct them to cause a disturbance, so that their deportation is stopped.

Another time, in ZRH, a girl started to undress while taxiing for takeoff and then proceeded to kiss the female FA who came to investigate. She was offloaded on the tarmac and taken away by police (also kissed the cop).