Welcome to the forum Lareda!
1. To place a house on the lot, you need to evict the family living on the lot first. Next you need to place your desired house down on the lot. Finally, place the family back on the lot and enjoy their new home! Make sure there is nothing on the lot by checking that there isn't a house sign above the lot in Edit Town. If there is, use the bulldozer in the bottom left corner of the screen. If there isn't an empty house sign, don't use the bulldozer.
2. Have you tried the diagonal wall tool? In roofs, it is the far right tab. Sometimes, the appearance isn't great, but nothing a little fiddling about can't fix.
Hope this helps a bit!