Yes, exactly. (And no worries about rookie questions. That's how people become non-rookies.)
In my case, I run the REPL using screen so I can go back to it for debugging if I need to. But you can also run "headless", without a REPL at all if you want.
Yep, pretty much. I've only ever dealt with low-load applications so there was no load balancer. You just put the Lisp server in a state where it refuses new connections (serves up a "temporarily down for mx" page, or reconfigure the ngingx front end to do that), wait for existing connections to finish, and then restart. It's no different from any other server.
In my case, I run the REPL using screen so I can go back to it for debugging if I need to. But you can also run "headless", without a REPL at all if you want.