> From what I can tell, the iMessage app gives the sender no indication of how many keys have been associated with a given iMessage recipient, nor does it warn them if the recipient suddenly develops new keys.
iMessage does warn you on all devices when a new device gets added to the account. Of course that's during normal operations. Unless somebody reverse-engineers one of the apps, we can't know whether the protocol has a provision for "please add this key but don't warn the user".
That's the thing with crypto in closed source software: it's completely useless because you have to trust the software vendor not to put any backdoors in.
And since two weeks, unfortunately, the only think we can trust is that the vendor actually DID add a backdoor.