I tested this game using a "previous version" installation of the steam version and was able to get a single player game running in LAN mode without steam or internet. I do have mods installed, so I will do a test of an installation of the current version, as well as an unmodded version, to see if those run without troubles as well.
Update: I've tested the previous version without mods and the current version without mods, both ran fine in single player LAN mode without steam or internet.