Ouch!!!
After guest and myself have between us identified no fewer than eight solutions, along you come and say that the intended solution is different again!!!
As you describe it, it doesn't quite work, because the first few of the mass of lemmings reach the last platform before the climber/floater (at least on the Mac). But this works:
Lemming 1 digs/mines the first platform and bashes the sloping wall, letting one go ahead. That one builds -- as in your solution. Because building is slower than walking, some lemmings reach him while he's still building. The first of them becomes a climber/floater and digs/mines to the exit.
You don't have a second builder to stop the bashing (you only get 1 of each), and anyway you need the lemmings to bash right through the wall so that one of them can climb/float. But on the Mac, this is still OK for time -- you finish with six seconds left.
We should have a poll, to see whether 99% of people really did do it like that....... personally I think my two solutions are much more obvious.