> but whether WhatsApp servers report to the device that the message has been delivered

It is hard to check what WhatsApp does, but in Signal it is not the server, but a recipient who sends delivery receipt. WhatsApp then has to either recognize encrypted receipts or allow only one-way conversation during attack. Carrying out the whole attack just to decrypt "hi, are you here?" is not really interesting.

The delivery receipt is the message that is directly sent after the message has been delivered. Not too hard to distinguish those from other text messages.

So they can recover the messages, right? However, wouldn't these messages still be encrypted? Sure, they force a key change, and the messages are encrypted using the new key and sent. Theoretically, an attacker could have multiple copies of the same message, but these messages would still be encrypted under a variety of different keys right? Wouldn't the content of the messages still be secure?

Unless the key-change forces the user to be using an insecure key-pair, but is that actually happening?

New encryption (public) key is selected by the attacker, so he knows the decryption (private) key. Basically attacker just puts real device offline and registers his own device.

Wouldn't the attacker need to be authenticated as the user of the real device for this to work?