So I had a few whacks at level 7 on Lemmini, and I'm really inclined to believe that we have a case where the level is possible on Superlemmini but not on Lemmini not because of the physics but because of the very strict timer and the way it works with the Superlemming gimmick. Here, you need to take into consideration that each bomber takes 5 seconds, and with all the bombers that are used to get through the extremely thick wall this uses up way too much time on the clock. Also the extremely long distance that the lemmings need to walk before they even reach the wall. Worse of all, the game clock is sped up along with the lemmings being sped up. In contrast, in SL the lemmings are sped up, but the game clock goes at the regular pace. Therefore, it is a complete waste of time to play the level on Lemmini as you're just wasting time on a level that isn't possible to finish on time. Obviously the only fix is to increase the timer, but then it also makes the SL version no longer as big of a challenge.
That reminds me. I do recall a snow level from Pimolems where the Superlemming gimmick is used, although I have recently been reminded of it from a few months ago. I have totally forgotten about it due to how much time has passed since I last played the pack on Lemmini. So yes, there's your confirmation that Lemmini does indeed support the gimmick.
note: I also forgot. The level had the problem again where the hatch and exit don't appear when loading in Lemmini, and so I had to resave the level in the editor. Also, unchecking the read only box allowed me to save the level under the name you gave the file