I was going to say raising Mayhem 3's time limit might make it too much just like Fun 24 (ie. the earlier repeat), but then again I guess the save requirements are different, so maybe it would be different enough to be workable. I think we should at least keep the time limit as tight as we can, to at least preserve that same feel as the non-reversed version. 5 minutes I'm guessing is probably too much in that case, but I guess I'll have to play around to find out how low it can go..
And speaking of repeats does make me think that, at least for the original Lemmings, maybe having the levels in original order is better than in reverse order, even though the latter sounds more thematic at first thought. I say ordering doesn't matter, but with so many repeats in the original set, having the order reversed means the player first encounters what is likely to be the harder version of the level, before later encountering an easier version that may well accommodate the exact same solution from the harder version. That probably wouldn't be desirable. (It would be as if the original levels have "Just a Minute Part II" come before "Just a Minute", though granted maybe not the best example.) ONML and the rest though don't have repeats and the "reverse order" idea may still fly there.