I've just developed a grid system/ quasi dark swarm firing system that can be used to use abilities on screen. For example, in my test map below, when you cast dark swarm to your left, it says option A chosen, or if you cast it to your right, it says option B chosen. Up and down could also be added with ease, as could diagonals by using brings to above/left etc... It seems to me that this sort of interface would be easier for people to use, as they don't need to ever leave the screen where stuff is occuring (The defiler follows the hero, or IS the hero)
Its a bit hard to explain how this works, so just play the map. (Note that I have no idea why its centering to the left and the right, it must have something to do with an anomoly in the grid system, as I triggered it to be above/below (hence the text, you have chosen the above/below, as opposed to left/right).
From the looks of your triggering, you did trigger it for left-right detection. Note that if you create units at a location, the shape and size of the location has no effect on the way the units are created.
If the grid system were a little neater (currently any dark swarm cast quite close to the defiler counts as option A, whether above, below, to the right or left), then this system could be made quite useful.
The way I'd set it up is for a menu, either a text message or a grid of units, to appear so you know where to dark swarm.
Yeah, I know its far from perfect, as I have not yet even finished the concept map out of lazyness, but I thought It could be pretty cool if used well.