Reading Events -> nostr client need to pull from relays u select
Writing Events -> publish and also can republish ALL events under ur npub (slow n long)
relay server ur client was connecting to IF down doesn't matter really - when backup it will auto resync messages from other relays servers (impossible that an event will reside in all 300+ distributed relays servers ) what logic n NIP guides replication of event between relays - not too sure for now.
if you OWN/ADMIN a relay server is it an one among 300+ no hierarchy same like btc node p2p - ur publish notes propagated to other relays
without ur permission