To answer your second question ( I have no experience with the KS+ level editor ) to know for sure how it works there... Okay, I do have it, but it was so long ago and not used often enough to recall how it works there...
Your second question though, you have layers, represented by the buttons 1 to 8. 4 foreground layers and 4 background layers. Clicking on the number with left will select that layer. Clicking with right will toggle its visibility. By making all layers invisible, then toggling a layer on will allow you to find out on what layer an object is.