I decided to try Sunsoft Level #30 (Fall an No Life: Part Two) and tried a slightly different solution, and, lo and behold, it worked!
(A little notice; you may have noticed that I used two bridges on the tiny gap; I found a way to use one builder and still let just one lemming out, so, I mined and used the final builder to keep the last few lemmings from taking too much time.
The main trick i used was bashing
under the one-way blocks, and as you will see on the other pic...
manuevering the lemmings, using builders, to juuuuust barely inch their way through the tiny gap, onto the platform, and they all get it with about 4 seconds left.
Perhaps the most evil backroute in history.