It was the very first thing that came to mind, precisely because you clearly don't have enough skills to go up the left side (It helped that the trick was fresh in my mind as I had played another level with the same trick last night -- "Coalburner" from Lemmings Plus I.)

(for an example where the harder solution is disguised, take a look at "Singularity" and its repeat in Armageddon, "Duality")

I think we shouldn't get too far ahead of ourselves in terms of looking at the later levels when the early ones still need attention, but since you brought it up, I had a look at Duality.

I'm not sure what you mean by "disguised"; it seems very straightforward.

Could you attach the problematic level file so that I can maybe determine what went wrong?

You're lucky I still have some of the old levels I did for testing SuperLemmini 0.103b. Here's one of them, it should have the same problem.

It is true that the one-way arrows are intended to be steel.  I'll see if I can cleanly add in steel plates for Perfect Harmony.

For Song of the Lemming, I would argue for keeping the left-facing one-way arrows because putting steel there might make the level a bit too hard for Picnic, what with trying to navigate all the water pools, plus it would make the best solution too similar to its repeat (for an example where the harder solution is disguised, take a look at "Singularity" and its repeat in Armageddon, "Duality").

I have the raw files for Revenge of the Lemmings somewhere, given to me by möbius some years back, but I don't know which hard drive they're on.

Also, part of me likes to have a time limit in every level for this pack, even if it is far more time than you'll ever need, because it's meant to have a traditional feel to it, not least because of it only using the original 8 tools and no switches, zombies or neutrals etc.  Also, it disguises the levels that have a genuine time crunch to them.

UPDATE: Here's Perfect Harmony with steel instead of the mixed one-way arrows, and what I think is the intended solution.  I also shrunk one of the water pools to make the solution fairer (otherwise the second miner would be one pixel away from death).

For "Song of the Lemming" and its sequel, if the dual one-way arrows were intended to be, in effect, a steel area, why not replace them with actual steel now that we can do that?

I feel strongly that because this is a "best-of" pack, we should limit changes to ones that make the same levels work better, rather than redesigning them into our own levels based on the originals. For instance, unlimited time and steel in the Bubble set weren't available when these levels were made, but it's a reasonable guess that the designers might have used them if they had been. We can fix backroutes that the original designers may have not fixed because they didn't find them. But WillLem's proposed changes are too drastic. If there are any levels that can't work without drastic changes, we should accept that they don't belong in a "best-of" pack and cut them.