> If WhatsApp tries to backdoor a channel and one of the users has key change notification, they'll find out about it

The problem is that the might not find about out retransmissions. You are trusting that the "double-tick" means that the message won't be resent, but presumably WhatsApp can indeed retransmit those messages with the new key under pressure from a state actor.

They need to specifically address this point; it's the only thing worth talking about. The rest is just a discussion of implementation of key exchanges generally.