"But the structural and legal barriers set up to forming a new party, by the rules created by the Democratics and Repbulicans, are humongous. If anything, the viability of a third party has been going down (Perrot -> Nader -> no independent candidate getting anything meaningful)."
Yes, Ross Perrot came as close as anyone has, but he had no viable party behind him. If there were as many dissatisfied Republicans and Democrats back then as there is now perhaps there might have been enough momentum to have formed a third party. Unfortunately there seems to be few strong leaders to build a third party around today.
"There is no way this occurs. Will he put up a stink if there's controversy around the outcome...absolutely. But it will likely be in the form of an Al Gore. Litigate and contest things for a bit, and then move on. I would not be surprised to see this regardless of who wins. But pretending he won't voluntarily leave is just an absurd media talking point."
Hope you are right. If he legitimately loses, I have visions of the military grabbing him by the scruff of the neck and dragging him kicking and screaming out of the White House with Bill Barr running behind him yelling "Stop, you can't do that!".
My real concern is that with the situation with the post office, mail in ballots and Trump now encouraging people to vote twice, that there is going to be so much confusion over the results that this election's legitimacy is going to lead to a situation where no one really knows who won the election.