Good rule. The bracket part is about faking and hiding the network lag of replicated information.
For the Session Part:
I saw this question a couple of times now and also already talked with some others about it.
As far as i understood, the normal Session of UE4 can’t do this. They are stuck to 1 Session with 1 Map
at a Time. So not even your Clients can load another Map then the Server while being in its Session.
So i guess you will need to handle this in a new Server System, out side of the UE4 Session System.
Correct me if i’m wrong.
I heard Raknet is a good way to start this, since it is free and you get some kind of Instance System with it.