Point/question #4 can be answered with the availabilty of a replacement flight. Try to buy someone off of a flight very close to a very expensive rare event...the availabilty of "open seats" on other flights is zero, the airfares paid were huge, and the rarity of those events is supreme.

This will be an exageration of a hypothetical situation...Cubs win the wild card, make it thru the playoffs, and are on their way to New York for game 7 of the World Series. Cub fans have been waiting for a hundred years for this shot, and the lights are packed...the only person you are going to be able to buy off that flight is a St Louis fan, and he would most likely ask for some crazy compensation.

Now that is extreme, but thats when you get the non-voluntary denied boardings...low frequency service, with long duration until next open flight.