And I thought there was only one solution to this level! I first tried a solution very similar to Ahribar's, but I was always about 30 seconds short. The route I eventually found to work is very similar to yours, Gunpowder, but different at the end.
Lem3 bashes through the steps. Lem4 builds to the left to the OWW. Lem1 builds over the narrow gap. Lem2 mines on the OWW, turns to the second narrow gap, and bashes onto the room. Lem1 does the same thing the hero does in Fall and No Life. Lem2 mines out the room to the left, walks up the stairs, and turns. Make him build a step or two back from the very edge of the top step (so his first step can block the path of lemmings walking up), and make him bash through to release the others. Finally bash the remaining OWW.