Adventures across the 3rd dimension

After reading through the post again, is the shadow appearing only where the extra triangles were added?
If so then it's might just be a topology issue. It'd be interesting to see the wireframe of it in Blender just to see if you can relax the internal edges somehow.

The wooden wall didn't get any weird triangle shapes. But like the stone wall it does have a "trim" of shadow around it and I'm pretty sure there's no geometry to cause it so I guess they have indeed managed to do something with normals or vectors or something else mysterious, to produce shadows. And I messed them up on the stone wall. But I've no idea how they're done so also no idea how to fix them.

And also, when I added collision to my wooden wall (which reminds me I should have changed the collision material from stone to wood... ) I noticed that the chunkmerge thingy had reverted from "static" to "clutter" template which means it might have used the wrong template last time I changed the stone wall, which means I'll need to redo it just in case... Or wait and check it in game, I guess... If I can kick it off the building, it's set to clutter...

It's a flat surface so I doubt it's topology, but yep it's only within the area I've made changes to.

Edit: Here you go:
 

Actually, that "edge" around the wall shape is where the shadow is, and in my last change I removed the "edge faces" along the wooden column in the middle, to enable the "center" vertex to be moved - and avoid very very slim triangular faces. So I guess that's got something to do with it, although the shadow was obviously there before I messed up that edge...

It will be topology, even if it's flat. Vertex shaders don't really like long narrow triangles so it may be worthwhile to subdivide those edges or maybe slide the apex vertex (EDIT: that's probably not the right word, I mean the corner with the smallest angle) closer to the window.

It would be a lot easier to try various things if I could see the shading in Blender and not only after I have passed the file through a couple of other apps and moved it into the CK...

But maybe I can learn something tomorrow...

Do you mean with the textures as well?

I tried to recreate the topology with Blender's smooth shader and I get those darker areas in similar places too...

 

Moving the small angled points did reduce it a little bit but they were always there in some form. You'll probably have to add some extra vertices in the flat plane and reform some of the triangles to get rid of it completely.

I could see a faint shadow originally, in Blender, but not anymore. It looks completely flat.
I can't view it with textures in blender because I am afraid of messing up skyrim's material settings.

I had an idea last night while trying to sleep.
When I was trying to read stuff about Nifskope earlier I noticed a few references to "smoothing groups" and I thought this sounded a bit like Blender's smooth/flat shading options. And then I wondered if that was what all those unconnected double vertices were. I initially thought that they'd be a glitch in the import/export process but it would make more sense if each separate piece was a single "smoothing group" to calculate the shading and the double vertices were deliberate ways to make hard edged corners.
If this is true then you could create this by selecting all the vertices in that loop where the recessed window connects to the flat wall and using the Rip Vertex function from the menu.
This would unlink the window from the flat wall so the shading wouldn't calculate around the corner and then the topology of the flat wall would be irrelevant.

Maybe.

You need to reduce the number of edges going to single verts, you have the star above the window and the corner at the bottom where several edges converge. If you redirect some of those edges it should sort out the problem.