I just wrote this NIP, and specifically motivated by things like that. Although they cannot ban math (so they cannot avoid people using Nostr), they can associate pubkeys with personal identities. So if they look at relays they can figure who is posting what. 

https://github.com/nostr-protocol/nips/pull/1595 

 "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.