I can see how you would leave the receiver in the dark by sending them the original, deferred message, but how would asking the sender's device to resend with a different key not result in a notification?

Furthermore, as soon as the sender attempts to deliver another message to the recipient, they would get another notification (because the encryption key changed back to the real key); alternatively the attacker could continue blocking (and reading) messages to the recipient, but the lack of delivery would be noticeable.

You could escalate it into a MITM rather easily, though, by attacking both ends; but again, a key change notification should be displayed to both parties.

Assuming the closed sourced app works as advertised, obviously.