I think it's actually perfectly okay that Nostr only does CRD, and not U. That's a valid system design choice. From early on, however, Nostr clients have tried to make events "editable", which requires abstractions on the client side, since the data itself is fundamentally immutable. Presenting immutable data as if it's mutable presents some risks, and some Nostr apps handle that presentation problem better than others.

I'm writing a relay in Elixir—so yes.

Replaceable events are identified by address (as distinct from ID). You can have multiple different IDs at the same address. Clients and relays have to choose how to handle this case. The most typical approach, of course, is to display the most recent event, but other approaches are possible. The events themselves (as uniquely identified by ID) are still immutable.