Look, if Apple is the adversary and willing to lie under oath then there’s not much anyone could do. They could sign and push crafted firmware to a target device, game over.
The question at hand is, if you trust that iMessage/iCloud works as documented, how does an adversarial third party cloud affect it. So far it looks like iMessage and keychain are still secure end to end encrypted. Apple never had the keys to those to begin with.
Actually iOS does notify you when another device has joined your trust circle. So if you pay attention to that you’d know you’re being monitored. Suppressing that would need a crafted OS pushed to the device.
iMessage is very secure. The real question is what would China do if it takes off (no one uses it there).
The question at hand is, if you trust that iMessage/iCloud works as documented, how does an adversarial third party cloud affect it. So far it looks like iMessage and keychain are still secure end to end encrypted. Apple never had the keys to those to begin with.