PoSA is entirely trustless and solves the Byzantine Generals problem.
The Byzantine Gernerals Problem relates to establishing a dependable protocol for communicating over an undependable channel.
1. The problem is specific to 2-way communication. There are absolutely no arrows returning to the sender in the flow chart
Read the arrow. "Tells Joe to create a PoSA address
and receive it from him." So...he sent a message to Joe and Joe sent something back. That sounds like 2-way communication.
Edit: This also happens with PoSA node 1. So your two-way communication appears to happen twice.
What? That has nothing to do with anon.
The problem of sending secret information over a clear channel was solved long ago. Google "Diffie-Hellman Key Exchange". You probably used it to log into your bitcointalk session today.