Euhm, no, it would have access to any message you can see from the phone, old or new. Plus I don't think there's a per-message key at all. So they could have lo

Euhm, no, it would have access to any message you can see from the phone, old or new. Plus I don't think there's a per-message key at all. So they could have logged the messages, then install a patched iMessage on your phone to send them the key. That is assuming Apple's claim is true at all, and they need this in the first place.