So uh...just to be sure, not used to these types of games...offline play = online private game? Or is there actually an option to make the game fully offline aside from blacklisting the app in a firewall? I mean, I can do that, but if it wasn't an option I don't know if the game will have a panic attack or not.
Only way to Fully play Offline is to use offline mode for steam, or disable your internet adapter
Otherwise, choosing Private and selecting 1 player when going on quests is the only way.
Just don't use the cheats with other people and you'll be fine.