> Like they could edit the article?
Yes. An edit produces a new event.
> Or the relay could delete and re-use the same event ID?
No, an event ID is generated by the sign fuction, with the user priv key, applied to the whole payload, so only the user can create an event/id. If the content changes, the ID cannot be the same,
> Or is naddr sort like a hash that is permanent?
A sort of, it includes the "coordinates", check:
NIP-01 https://github.com/nostr-protocol/nips/blob/master/01.md
"for kind n such that 30000 <= n < 40000, events are parameterized replaceable, which means that, for each combination of pubkey, kind and the d tag, only the latest event is expected to be stored by relays, older versions are expected to be discarded."
NIP-23: https://github.com/nostr-protocol/nips/blob/master/23.md
"These articles are meant to be editable, so they should make use of the parameterized replaceability feature and include a d tag with an identifier for the article."
...
"The article may be linked to using the NIP-19 naddr code along with the a tag."
So "pubkey, kind and the d tag" are the coordinates used to get the last version of a replaciable event.
To inspect a nostr object use https://nak.nostr.com