> It could also pass back Bob's answers, and so Alice could have what appears to be a normal conversation - except that Alice only sees single ticks, instead of double blue ticks.

No, it can't do this, because Bob's answers contain the "delivery receipt".

Hence, the attack doesn't work on conversations.

EDIT to reply: messages are sequential and "delivery receipts" are messages, so it would be visible if the attacker dropped some but not all messages. AFAICT.