>iMessage is fully E2E - Apple does not have any ability to decrypt messages, only the recipient can.
This is not true.
Apple is deliberately deceptive with their claim that iMessage has "full E2E" as you put it. While it's true that Apple can't decrypt iMessage traffic in transit Apple can absolutely decrypt your messages.
For a simple explanation, think about how an iCloud restore works for a brand new iPhone that you've just purchased. All your messages magically appear on the device! But they were all encrypted and Apple claims to use E2E encryption - how is this possible?
It's simple - iCloud backup also backs up the encryption key used to read your messages. A restore wouldn't work without it. So in practice, Apple is deceptive when it claims that it can't read your messages (or give them to law enforcement, etc.) - if you are a normal user with iCloud backup turned on and you haven't explicitly taken the extra steps to disable message backups, Apple can read every one of your "E2E" messages.
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).
That’s true, though IIRC Apple can enroll new devices into any account, so they can eavesdrop if they want (but not decrypt anything from before they started eavesdropping)
iMessage is fully E2E - Apple does not have any ability to decrypt messages, only the recipient can.
There's a full white paper explaining how iMessage works, and how it achieves full E2E. Same for FaceTime. And iMessage group messages.