rickycook responded to this part with a proposal, but I have my own take on some of it:
For the "tied to a unique piece of data," that's why I want Bob to download lots of messages, hiding the fact that the person at 14.85.101.86 is the user with the recipient ID of ntULzh2AeEgPH9bKxrn3gUL. Bob should also be rotating his IDs all the time. Maybe they're single-use. And if Bob wants Alice to be able to send him messages, then he (out of band) has to give her a huge list of IDs he'll be watching for, in sequence. If they arrive out of sequence, he knows to be supremely suspicious. Also, yes, I recognize that key management is THE PROBLEM. And I'm essentially inventing dead drops. But in my defense, I'm trying to come up with a way to make it easy for a lot of people to use, thus making it easier for everyone to hide in plain sight.
For the "guaranteed expiration," I am actively assuming bad actors would download and archive all messages. I only propose a limited number of days to lessen the storage costs.
For the rotating keys, as I understand it, there's Perfect Forward Secrecy, but it's very chatty (think of it as "online"). There's also a weaker form of Perfect Forward Secrecy (think of it as "offline"), but the risk is that if the communication if broken at any moment, then you can't recover from within that channel - meaning you'd need to go back to the person out of band, and restore communication. I'm probably summarizing it very poorly, but my mental model for it is roughly, at the end of every message I send you, I tell you what new password I will use when I send you the next message. It's actually way smarter than that, as I understand it, but that gives me enough of a mental model to work with it as a lay person.