Hey I came back to say I fixed it. I went and repurchased the game on GOG, even though I purchased the game from TW directly like 8 years ago or something. I didn't really care to try to find the activation info, so I was using a torrented copy for this cause I thought it'd be easier. So I tried this on the new GOG copy and it works. I heard the Steam copy isn't great for a few mod tools or something so I'm avoiding that. I'm not sure if this is the actual issue or not; all I know is it works now, and I'm on Windows 10 and using it with Warband. Was able to load up the Viking Conquest map on the first try after obviously changing the "Mod=" line in the config.
Weird thing is I didn't have to do the Warband workaround linked to in the OP like I did before? Just dropped the .exe and folders in the root GOG Warband folder. Like, am I gonna run into issues or should I be good if I can load the map up and tinker just fine? I did try to save a map change and the game wouldn't load up, but the map editor warned me when I saved that it was likely because I raised the ground over the top of a troop spawn or something to that effect. Curious if that's the reason for the Warband workaround.