[olliej]

if you disable iCloud backup, apple cannot decrypt those messages.

iMessage is 100% e2e. There is no key compromise. Even the backup does not compromise the keys you use for iMessage.

As for restore from backup:

You need a separate device to approve the attempt - and that provides the necessary data (which is also E2E), for your device to decrypt your iCloud Keychain, and to add its public key material so it can receive new messages.

In the event you have no devices left, the fall back is the iCloud Key Vault, which are in essence a set of HSMs that apple cannot access (this is covered in their security white paper, and a talk by Ivan Krstic a few years ago at blackout).