Hi,
Usually I make the slab on the interior side of the wall so it doesn't shows up in the elevation, and the position of the columns (at least by us ) aren't in the real construction either in line with the exterior side of the wall so they neither shows up on the elevation or 3D
In the elevation you have the possibility to make it a drawing and to edit it
In reference to the 2nd question, you can draw a wall with 0 hight
Regards, Ede