Re 5. The passkey is used explicitly for the wallet.
Re 6. A regular user doesn't store his passkeys in his desktop, and the passkey stays in his phone or physical key and only the prf result is sent to the desktop.

1. The client can regularly check the relays and republish the list of salts if they are deleted.
2. Every vendor can setup their own relay.
3. The list of salts is backup-ed automatically by ios/android because its in the app data and is not "secret".
4. The same list can be exported by the client to a simple text file if the user wants a wallet independent backup and doesn't want to run any app using this protocol for a long time.
5. Passkey rotation is generally not needed because the secret part of the passkey is not supposed to leave the TEE except when migrating from one vendor to another (using a secure protocol like CXP). In the exceptional case of a passkey compromise, the user can always move its funds to another wallet using a new passkey.
6. The UX cost of using iCloud/GDrive is very high (login, vendor auth)

The primary salt is an hard coded string defined in the protocol. Using it the list of the salts used by the same passkey can be obtained from the relays.
If by "the user trusts his nostr relays to store the salt", you mean to trust the relays to not delete them, trusting iCloud/GDrive is not better.
Also, Breez intent is to mitigate the trust by handling a relay dedicated to store the salts.