I see no fundamental difference in outcome from miner collusion in scorched-fee (which isn't guaranteed to pay the "right" pool!) and miner collusion in knowingly mining a doublespend transaction.

Well, they're the same thing. Replace-by-fee is miner collusion in knowingly mining a double spend, just triggered in a certain way.

Remember that you aren't paying the bad pool, the bad pool is paying you. Whichever pool benefits from the scorched earth protocol can simply pick an address out of the transaction it perceived as starting the protocol, and pay that.

Zero-conf needs something else for security. A guarantee it can not be doublespent in the relevant time frame.

I think this is the core point which many of these debates revolve around.

No payment system provides guarantees, though some are stronger than others. All they do is manage risk.