Agree those are potential fixes for NIP17 DMs, albeit they rely on all the things you mentioned. How many people are going to exclusively use auth-enabled relays with IP access alerts? And those alerts could also work backwards and inform the attacker of the true holder's IP, which needs to be accounted for. I mean it's all doable but these dumb relays sure are getting pretty smart.
You do have a way of knowing: if your DMs are going to a semi-trusted relay and you have to AUTH to it in order to read them (as you should under NIP-17) that relay may inform you about who has read them, when and from what IP address.
If we implement https://github.com/nostr-protocol/nips/pull/1647 that will also kill the attacker ability's to read anything in the first place unless he wants to reveal himself by registering his bogus device or publishing new encryption keys.
Discussion
Clients should nudge users towards using such relays for DMs, and that is happening right now in every client.
Relays were never meant to be dumb in the sense that they wouldn't have any internal logic, that was an unfortunate choice of words and a huge misunderstanding that damaged Nostr a lot, it's a surprise to me that people could have thought a relay would be really that dumb, do no filtering, no access control etc and Nostr would still work? Nostr would never work if it was like that.
That's fair.