I don’t consider it a waste of time at all. I am using that very script myself and my project is top down isomorphic as well, so I was wondering what the issue might be in case I run into it myself.
The buffer visualization being all black is expected until you mouse-over an actor while a PIE session is running.
As a player’s note, I kind of like the issue you have above. If you highlighted the entire unit it would make it extremely difficult to see the obstructing unit (at least from that viewpoint).