Is there a way to run the base game without any mods in 2560x1440? The game does offer the resolution ingame, yet it does not actually set it - and when I look into the ini, it also does not put it in there. Am I right in assuming that it is unable to diplay that resolution, or am I overlooking something...?

I haven't been able to do this without a small amount of modding.

There are two sites I always check for this kind of thing: PCGamingWiki and WSGF (widescreen gaming forum).

In this case, PCGamingWiki links to a patch that, along with dgVoodoo, appears to work. Though it does make videos and the main menu display a bunch of weird colours for me, the in-game menus are fine.