I'm aware of the problem.
I'll ask you a last time, why does my suggestion not work?

Your suggestion would work, but I don't think It would work very well on my map. Considering there are two reaver levels, just about any air unit I use would go too fast, leaving them to do, just about what happened before...
I do think your idea is creative, but Im not sure it would function well on this map.

Well I admit it requires considerable effort and probably a lot of testing.
But the too fast air unit problem can simply be solved by stopping the air unit if it gets ahead too far.

You can use cloaked wraiths, center a location around the wraith that is 3x3, then when there are no units in the area, order the wraith to stop (move it off the map, for one). Once they get back in the 3x3 area, order the wraith to move to the end again. Rinse and repeat.

I bet you will run into unit limit problems, if you try to have an air unit above every runner. Your path is to long to spend about 250 locations for the path.
-> 1 of the 2 solutions could be used:
Solution 1 (maybe a laggy one):
You use a huge mobile grid system which follows each path and ordering the units step by step. This stops you from using air units and ground units as runners at the same time (not to bad, or?) and it can happen that you create big lags with that system. Unit walk in a straight line, if you order them to move only a few tiles away.

Solution 2 (the easy one):
You use null tiles (the black ones) as the path's borders. You are able to build on it (and create and move units on it, if needed, if you use the right ones) and the enemys cannot cross the null tiles. Ordering ground units works with 1 trigger action.

Not for every runner. Lol. Just for a group. At least 10 runners.

I like your Idea Alhi, I have two questions:

1. I tried this out and got an interesting result, When the build was half on the null and half on the High Ground, the whole structure was red(can't build), however, when half over the path(cracked dirty) and half over the null, the part over the path was red, but the half over the null was green....

2. Can the workers(Probe & SCV) go over the null?

Only walkable null can be moved over, and that's only when moved, created, preplaced, or dropped on it, nothing can just walk on it.

If he uses air units as one of the spawned units, they will be able to go over the null, and possibly, fly out of line.

No, the air levels work perfectly, its only the ground levels that step out of line(forgive the pun)

What's the pun?
I don't know if this will work, but since the path generally leads to the center, you could detect when they're off the path, then order those who are off the path to move to the center of the map. When they are on the path again, then order them to move to the corner they were supposed to go to.