Yes, it is buggy as hell
Far as the combinations/textures issue and the moss sidedness. The way i've generally fixed this is to flip wall pieces. If you have a piece that just will not match, flip it.
Never had the lose W sign problem. But you should be able to click your walls from the wall panel on the left like a trigger or item instead of using the W.
Roof with missing pieces? Same thing, flip the wall piece.
Far as making the roofs, maybe show some sidebars screen shots and screens of your buildings? Which root template walls are you using?
In the larian video it suggests you copy your wall and paste it, then change the root template. This causes me nothing but problems. When I take a little extra time to make a matching wall/roofpiece it works better.
I was able to get my little town to match up pretty well:
http://imgur.com/bXoTSeGhttp://imgur.com/3fI6rtVInstead of making a T junction for interior splitting walls I usually add another wall and simply put it inside. This helps a lot with the textures issue too.