Earlier quoted context omitted.
> end-to-end only device to device (so I can't swap seamlessly from a desktop to a mobile session) How do you accomplish a seamless swap from desktop to a mobile session using end-to-end encryption?
iMessage has end to end encryption and supports multiple devices. Each device uses its own keypair and when a message is sent, it is sent encrypted for each of the recipient's devices. https://www.apple.com/business/docs/iOS_Security_Guide.pdf
If it's because Apple tells me so then I think can see that the end to end encryption is broken and interception is possible.
Are old messages re-encrypted with the new key?
This would be even worse.