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.

You can beat the rap, you can't beat the ride.

Anything that causes a delay can be a potential safety risk

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.