Answer: Not retarded pathfinding and let those smaller units squeeze by you inconsiderate fatty longer-range units.
I don't understand why the fatty long range units wouldn't just move.
Step out of the way, let the troops go through, step back to where you were. Makes perfect sense to me.
Yes, of course, this might very well put your long range guys a bad place... But that's why you micromanage them. If you're just trying to get some troops from one place to another you really shouldn't have to tell individual units to get out of the way.
if wall-ins are compromised then just make it so units told to stay in place won't let anything push them around and squeeze through, or just allow it for allies only.
I fail to understand why a wall-in should even really be possible.
Ok, obviously, a normal human isn't going to shove a tank around... But if we're talking about a pile of zerg pushing up against a pile of marines it isn't like the marines are fused to the bedrock or anything. It makes sense to be able to shove people around a bit.
Sure, if you've got sufficient numbers, you should be able to substantially impede someone. They aren't pushing against a single marine, but rather pushing through a group of 30 or 40. And at those numbers you're going to cut down a lot of zerg before they get too far.
But why should a marine be treated like some immovable object?