t0b

Good idea of extending the technique.

I want to address you last concern. moxie's specific concern is that without the extra measure I explained in the blog article, WhatsApp could find out on a large scale, covertly (!!!), who has this setting enabled or not. Simply because after a while pretty much every client will have to face the decision whether or not to retransmit for a benign reason.

Of course if they MITM, they can distinguish the re-send text from the garbage text, but the point is that then they are MITMing already and risk being detected. So instead they could've just MITM the conversation from the beginning on with risking of being detected.

response: https://tobi.rocks/2017/01/there-is-a-whatsapp-backdoor/

you get "delivered" and "read" notifications before the recipient sends you a text response, so they must be independent.

seems like it, yes

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.