Now, posting as just a regular user:
I agree with Icho 100% here. Hidden traps, fake steel, they were amusing during the early days when someone was the first person to come up with the idea, and back when designs that mislead you about what's what were considered a fair method of challenge. Nowdays, the majority of players find them annoying, and as Icho said, this is even more true when using an engine that doesn't allow manual rewinding (ie: Lemmini or SuperLemmini), or anything like clear physics mode that shows you in a clear way what's what (I'm not sure if even Lix has this feature yet, actually, though I'd imagine it won't be far away if they don't already have it).
Also, you mention wanting to be able to make a level where "one lemming sacrifices himself to unlock the exit", and you've come up with a very roundabout way of doing this - the implication I get is that there's a trap, and the trap animation reveals the hint. What stops the user from figuring out the hint, and restarting the level and reaching the exit without going via the trap? You should strongly consider switching to NeoLemmix; NL actually supports having buttons that unlock the exit. These buttons don't kill the lemming, but this is easily solved - place the button in an area the lemming can't get out of alive. (See Outrageous 11 "Relentless" from Lemmings Plus V for an example of this.) Not only this, but in general given the kind of designs you prefer making - while NL doesn't stop you from making them, and they're not likely to suddenly be loved just because they're on NL, at least - as Icho mentioned - NL will result in less frustration, because the user can simply rewind, or use Clear Physics Mode to see exactly what's what.