gamingrn: I've been playing Warband native without mods (although I do have the original Viking Conquest DLC, I've not really used it yet) for the last few weeks. Today I opened Galaxy and it updated Warband with a large (about 1 GB I think) download and I experienced the same problem you did. All of the graphics, menus, backgrounds, and so on seem to be from Viking Conquest Reforged. The graphics are goofed up in spots as well. It also gives the Napoleonic Wars DLC as an option in the loading screen drop-down, though I have never bought that DLC.
Not sure what's going on, but something fairly simple that worked for me:
Log in to GOG Galaxy and choose Warband (don't load the game, just go to its screen). Leftclick the slider switch button (to the right of the 'Play' and 'Update' buttons) at the top and choose 'Configure.' Then select the button for the previous version of Warband (2.060) or an older version. An update of around 64 MB downloaded for me then, and the next time I opened the game, everything was back to normal.
Note, Warband will still want to update to the currently (as of April 25, 2023) flawed version (2.061), so you may need to turn auto-updating off if you are currently using it. Avoid any updates until this problem seems resolved. No idea when or if it will be, but I hope so.
Good luck, and hope this helps.
This did indeed solve everything, must be the faulty update.
I had forgotten to mention the messed up textures (as I hadn't even noticed yet in my "What is going on?!" phase) but all of the issues seem to be solved by going back to version 2.060.