Use Damus, Relay settings are more obvious, & it shows what relays people use.

Ideally you should be using an Nsec Bunker/Key manager but I don't know any good ones on ios right now (but if I find one I'll update you).

No, you can use the same keys on any client (kind of the whole point of nostr), if one client is silly or stupid for whatever reason then you can use your keys to move to another client.

I would recommend exploring the ecosystem to see what experience you like once you get the basics down (clients on ios: Nostur, Nos, Freeform, YakiHonne) but for now start with Damus.