Actually, I don't remember which version I used when I created the level since it is quite a long time ago, but I checked the complete set and it is possible with CustLem2 too. There's a little trick used (nothing spectacular at all) to shave off a few pixels and get a lower fall that way.
I wasn't sure whether it was really necessary to use the trick, but regarding to Conway's comment it seems to be necessary for solving it using CustLem2 (I suppose you're on the right way btw, Conway).
As for the problem with mouse holding, I think (but I'm not sure) it never works if the next frame the lemmings is already doing a move no skill (except climber, floater, bomber of course) can be given to him (e.g. faller, jumper).
But also, sometimes it doesn't work if it is not such a case.
So, I don't actually know.