Re: "Ship! Come back!"
"If they tried to push an invite out to 5eyes, then each device could notice that the administrator has pushed an invitation to a new member."
Unless they update the app to silently ignore when certain keys are added to the conversation. When they control the client as well as everything in between, they can add eavesdropping in a way that would be invisible to the end user.
Of course, once they have the capability to do that, you have to wonder what processes are in place to prevent (eg) someone bribing a Whatsapp staffer to give them access to a private conversation.