Pain 16: The Pool - Yes, though you don't need that strict crowd control if you assign the hero lemming a blocker at the end of the bridge. I found Martin Zurlinden's Deep Deep Down (Pain 11) also really hard at first when I played that level, it seems impossible at first but when you know the solution, it's fairly simple.
Pain 18: Ancient Clockwork - That isn't quite possible in the original environment of this level in Lemmini, so you managed to take a little shortcut understandably, but that solution is on the right track.
Pain 19: Level Under Construction - All the intended solutions for Martin Zurlinden's levels are written in his document that comes with his pack, so I can say that this is a backroute, obviously due to that trick you used.
Pain 20: The Underground Secret - Yea this is correctly solved as far as I can tell.