Where do the corners fit in, I had a look through the files, yep, there are corners, they should cover this in the segment editor video, but they don't
Just use inside/outside corners.
? How do these fit in
2 wall segments in each direction (multimesh)
No idea here. In the segment editor video, they use 4 walls, but ingame I notice that it doubles them, even though you didn't put them in, so you have exterior walls, but in the SE video, it doesn't show putting exterior walls in, it does it automatically.
Ill re check the videos, but I'm sure no mention of the exterior walls which is what I am presuming you mean.
Also before someone says use signs or magic, my meshes are custom, they are UV mapped (as in unwrapped) rather than having a material assigned.