Just to answer that part of your question, and this might not be what you want, but I would have thought it more normal to pick the front-most object.
If you're only targeting the windows runtime, I believe the "Select Object" extension can let you do that in a single line. If not, you can use the fact that the "Spread value in alterable value" action assigns values in order from front to back. Something like this:
+ Always
-> Object: Set alterable A to -1
+ Mouse is over object
+ Player clicks
-> Object: Spread value 0 in alterable A
+ Mouse is over object
+ Player clicks
+ Object: Alterable A == 0
-> Object: Do stuff...