This is how I've adjusted the endgame portion so far, for Genesis with a sliding solution.
To test this, I got 70 lemmings out, then modified game memory to move all of them to x=26, y=33, facing left, sliding. The y part is arbitrary, the x part matches where my proposed sliding route should put the lemmings at. Starting from the frame where the sliding crowd reaches the top, it takes 752 frames before reaching the attached screenshot where you first see the crowd exit.
So it looks like I started the builder at y=12. 2 miners were used to get there from y=8, and since a miner lowers you fully within its first few update cycles in the stroke, I can relatively quickly switch over to bashing or building. 3 build steps are needed rather than just 2, since Genesis doesn't allow the lemming to get as high as you can in DOS. And it turns out speed-bashing doesn't really work at the top in Genesis (lemmings tend to jump as they reach the end of bash tunnel where the latest bash stroke is only partially there, and near the top, unlike at more normal elevation, jumping easily turns lemmings around).