Trick to adjusting item heights on walls?

Perhaps I wasn't clear. I already know how to do what you said. The issue is if the walls are down or partially up. Many times, the object simply won't slide up/down the walls If they're inside walls. The walls have to be completely up for it to work 100% of the time. This makes it very difficult when attempting to do this in a small room where vision is limited. I think a keyboard shortcut to move them up/down in increments would solve this issue and make building much better.

As for hiding windows/doors: It was like this in S1 and 2 (not sure about 3). Once you switch to buy mode, they disappear. It doesn't cause complications. If you can't place something because they're in the way, you simply can't place them. It's never been an issue. Not having them in the way to obscure your view makes decorating much easier. I haven't noticed this when using buy mode on a played lot, but it's happened when I've built on an unplaced lot.