If you were willing to use twice as many locations, you could have one the size of a block, and one a pixel. You constantly recenter the pixel of course. The block will be recentered to any unit in the pixel. If the pixel gets off, you recenter on the unit in the block.

Basically the only time it would confuse is when two hero lings (for instance) are colliding AND losing their pixel locations. While that may be a bit better, I'd say it'd usually not help much, though. If the same player is controlling both, they'd likely lose it at the same time, I'd guess. I would think inverted locations would be worse, though.

Also, one thing that I think is at least noteworthy to point out, is that since the triggers simply do not catch the units' deaths sometimes, if you actually kill the unit in triggers, it definitely will detect it that trigger cycle, but not next.

This. I think that's what I was thing when I started writing, but I screwed up.

Wait, is he even using a zergling? Chances are there's nothing to worry about...