For example, how can you specify how any coins received to an address may be spent without introducing CTV?
Fears about securely buying domains with Bitcoins are a red herring. It's easy to trade Bitcoins for other non-repudiable commodities.
If you're still worried about it, it's cryptographically possible to make a risk free trade. The two parties would set up transactions on both sides such that when they both sign the transactions, the second signer's signature triggers the release of both. The second signer can't release one without releasing the other.
And this is not only about multisig. This is about more general approach of unlocking things, by signing two messages by two parties. If you have OP_CHECKSIGFROMSTACK, then you can have two transactions: one on Bitcoin, one anywhere else (may be also on Bitcoin). And then, you can use the same signature, to release both coins, even if transaction IDs are completely different, and even if both are placed on different chains. It does not matter. If you can have OP_CHECKSIG on BTC, and OP_CHECKSIGFROMSTACK on ALT, then you can use a single signature to release both coins.