Take it from a lawyer: Do not argue with them. Pilots and ship captains are special. Their word is law. You can sue them afterwards, you can report them to police and they could be charged if they had no good reason to order your removal. But at that moment on that plane they are kings. If a pilot wants you off you have to get off. Once the pilot requested police help there was nothing that man could have done to stay aboard.
They should not have treated him in that way, they should not have been violent, but so long as the pilot wanted him off he was obligated.
Untrue in my jurisdiction and likely others. We have a limited range of circumstances in which the pilot in charge (or their delegates viz. cabin crew) may compel disembarkation. Drunkenness or presenting a safety risk are on that list. Overbooking is not.
So then one possible course of action in case of post-boarding overbooking would be free drinks until enough passengers are sufficiently drunk to be removed? One more for the endless list of less bad things that could have been done instead at the United incident.
That's isnt the same thing. The pilot can be incorrect. Him ordering your removal can be a criminal act. Him ordering you off could see him put in jail. There were cases about this decades ago (think white captains refusing black passengers). That doesn't mean that a passenger can stay once ordered off. Whether the removal order is legal or not comes afterwards.
Like trying to throw someone already boarded out of the plane, involving security officers, police, violence and broken teeth?
Yes, I would definitely say that that kind of behaviour was a really outrageous safety risk.
The pilot, accordingly to your narrative, should have thrown out of the aircraft all the crew and whoever was involved in this gigantic fuck up to avoid a very serious safety risk.
True, but a pilot can leverage that later power by not taking posession of the plane. Door stays open, airline cancels flight and everyone is now a trespassor and must get off.