I'll be entertaining people over the weekend and next week, but some quick comments:
For [Wicked] 15, I used four extra floaters before I could build a safe drop for the others. I used a digger to trap the others.
I verify that you need to raise the landing point by just 6 pixels, and the first lemming out can get 6 build bricks laid in time for the 3rd lemming out. Hence it's possible to make the drop safe with just a 2nd floater (this part is confirmed):
1st lemming starts building immediately upon landing, and 2nd lemming, after floating, will block on 1st lemming's 3rd build brick to turn him around to build left.
Because a blocker is used,
someone else would still need to check that there are enough skills left to finish the level and also to free the blocker and have him (and the lemming freeing the blocker) turn around.
Havoc 10 needs 3 more skills: 2 bashers and a builder.
2 is definitely enough: add 1 blocker and 1 basher. The real question is whether 1 basher alone is sufficient; I had a few tries last night but no luck so far.
Finally, for
Wicked 12 2 is definitely enough: 1 digger and 1 builder (remember you already have 1 spare builder out of the 7 given that you don't need to make the path to the exit). But I was quickly able to confirm that adding
1 digger alone is actually enough. See attached replay which shows how to trap and release the crowd (I'm lazy and did not bother with the rest of the level.)