For instance, if Coinbase had
contracts with 80% of the Bitcoin hashing power to guarantee their
transactions would get mined, but 20% of the hashing power didn't sign
up, then the only way to guarantee their transactions could be for the
80% to not build on blocks containing doublespends by the 20%.

This seems to be more of a problem with centralized mining than zeroconf transactions.

Speaking of, could we get a confirmation that Coinbase is, or is not,
one of the merchant service providers trying to get hashing power
contracts with mining pools for guaranteed transaction acceptance? IIRC
you are still an advisor to them. This is a serious concern for the
reasons I outlined in my post.

We have no contracts in place or plans to do this that I am aware of.

However, we do rely pretty heavily on zeroconf transactions for merchant processing, so if any significant portion of the mining pools started running your unsafe RBF patch, then we would probably need to look into this as a way to prevent fraud.

In the long term, I would love to see a safe, decentralized solution for accepting zeroconf transactions. However, right now there is no such solution supported by any wallets in use, and I don't think breaking the current bitcoin behavior for everyone is the best way to achieve this.

Adrian