No 1 is related to the ability of the client to search an user by nip-05. Because there is not a standard way to query a relay for a nip-05, the client have to to cache them or use an external service.
An user can have multiple valid addresses, but every client should show only the most recent one, querying the metadata (kind:0) event.
About the key rotation: because a nip-05 is a proxy to the pubkey, if you need to trash the old key and spinup a new one, you don't have to update it everywhere. Users that find your nip-05 (e.g. business card) will find always your update account. Users that already follow you will see an invalid nip-05, and they would be alerted.