➡️ “It seems rational actors will choose just to broadcast. […] Acceleration options are more expensive.”

That’s true, but I think you’re conflating two services. Well, technically, there are three services (fast, regular, and slow). “Fast” would be an acceleration service for those who want faster transactions and willing to pay for speed. Regular would be the normal CPFP transaction. “Slow” would be a cheaper, slower service for those who don’t want to pay higher CPFP fees and are willing to wait. 

A rational miner would offer a cheaper solution to obtain additional revenue not otherwise available. To them, it would be an option, one they could fulfill if they wanted. A rational user may be willing to wait longer for a solution that is cheaper than the default option if their transaction is not time sensitive. 

➡️ “So it would not be possible for secondary trading of options directly on-chain, unless you knew the counterparties you wanted to trade with ahead of time.” 

What if there was a registration step where traders sent the coordinator or AMM a small amount to register their address, and the contracts published the next day by the AMM would include these registered addresses?

I’m not particularly clever and I can see simplistic ways it might be accomplished. If I’ve learned anything in this space it’s that people are incredibly resourceful, and will find a way if the incentives are high enough.