I agree it makes more sense to handle a failure of 1) by using the PM to tell the server(s) to reconnect. Stopping and starting the SM's listening socket gives me a bit of a queasy feeling though; it could interrupt other on-going communication. Is the advantage mainly simplicity (stop and start the socket triggers the existing 6.3.4 logic above instead of requiring the SM to send a command to the PM)?
If 2) goes down using stdio if needed to coordinate commands to trigger reconnecting sounds fine. I don't know any reason why using stdio again would be a problem. The PM needs to consume each process' stdout and stderr anyway.
I don't think it's good though to try and route commands over stdio besides the ones needed to get the socket communication going away. Otherwise we face the problem of dealing with junk sent via stdout that led to our using sockets. One thing though is the PM can safely use each child process' stdin to send commands to that process.