Regarding the editors. The best option would to build in a menu to the game that lets people host a game & join the game (search for sessions, join sessions). That way you can start the server & client. Start the game with the server & do some things. Have the client find/join later. This will actually get directly to your points (a new player joining a session needs to update all of the state).
I can expand on the other points if the video above doesn’t clear things up.