"Nostr is decentralized, meaning that once an event is published, it exists permanently". I don't think this is true, if you post an event note between kind 10000 and 20000 only the last note should be reported by the relays
I don't know for sure. I don't understand Nostr really well. When a relay receives those events you mentioned they see it is associated with the pub key? If yes, we cannot assume that the relay is a honest piece and will no save that event for later.