What if I don't follow the person? The only evidence I had that the post existed was that someone replied to it. Do clients have a way to find out what relays are hosting a comment? Is that even included in the Nostr protocol?

And what about if I don't even have that much? Is that still just a client problem? And you didn't address my complaints about relay transparency. Is there anything that could substitute for this lack of transparency in relays and a protocol that doesn't facilitate such relay transparency?