I'm a lawyer. In my view its pretty clear contract law. A contract is formed once CO gives away your seat. If you can not get on the plane anymore then CO has accepted contract.

So if you volunteer, then read form and refuse to sign it, it depends on CO's response. If they tell you that tough luck your seat is gone, then I'd say CO already accepted contract and these terms that you refuse to sign are not at all part of the contract. If CO says that you need to get on plane since you refuse to sign then no contract since CO has not accepted.

Once CO takes away your spot on the plane contract is formed and nothing added afterwards is valid. If you went through the kiosk though then everything is different because you agreed to the terms long before CO accepted the offer.

CO taking your seat is a trickier question. I would rule that giving up the assigned seat is valid consideration and manifestation of acceptance by CO and a contract is formed. However I could certainly see it being ruled the other way.