M. Dilger on Nostr: Yes I think leaking the conversation key would expose the binding between the seal ...

Yes I think leaking the conversation key would expose the binding between the seal and rumor, not provably, but it would be highly unlikely to find a different conversation key that decrypted into a sensible rumor so effectively. And the conversation key is post-HKDF so it doesn't expose your nsec. This would have been great feedback for NIP-44 development.