lite client works fine for public messages... but private messages doesn't work so well.

Adding metadata back to the private message is bad. It's worse than a centralized system, because at least only the center knows who you are talking to. In a decentralized system without metadata privacy, everyone knows who you are talking to.

How about: download all private messages from your direct friends. discard messages you weren't able to decrypt. friends of friends arn't able to send you a direct message, that's okay. or if a friend is also in the thread and they reply, you could pull down those messages.

But actually, I think we can actually get the full peer good enough to run well on a phone, and this is by far the most worthwhile goal.