Apparently I only included a single screenshot in my
posted solution, and it's been ages so I can only reconstruct the solution based on that (and certainly don't remember how easy or hard it is to execute). Based on the screenshot, it looks like I probably have the first lemming from right entrance start stacking, and let only the second lemming from right entrance get past that stacker to go build. (It's also possible maybe the first one builds and the second one stacks, which may require picking the precise right spot to start stacking so no one overtakes that stacker.) Once the right-entrance crowd starts getting flinged to the exit platform, looks like I have one lemming start stacking to make the fall off the builder bridge non-fatal, and also quickly have another lemming club-bash to both get through the mushroom as well as make a gap at the bottom with the second stack.
It seems clearly important to get the stacker at the top to start stacking as far right as possible, to produce the largest amount of walking (as delay) for the left-entrance crowd, as well as the smallest amount of walking for the right crowd (so they can reach and start the other stacking as soon as possible). The timing and positioning of the builder and second stacker seems basically fixed. So actually I kind of feel like there shouldn't be that many things to try and fail on once you have the solution idea. Maybe the placements required for the skills are a little too pixel-precise? I can see that maybe having to line up the builder with the second stacker by eye is not ideal.
[edit: thinking on it more, maybe the extra early lemming you're having problems with is actually the first stacker? If a separate lemming builds, I wonder if the first stacker may be proned to overtaking the builder once he finishes stacking, given that I think stacker finishes faster than builder?
If that's the problem, I think you need to ensure the first lemming from right entrance does the building. Or maybe even that he both stacks and then builds. Either way would require the stacker to pick the right starting location (assume there's one) so no one will overtake him. I'll grant you that if this is necessary, it's a little inelegant. Then again, given the minimum skillset, it should be a logical tweak to try for getting the solution to work.]