CCExplore, I believe, modded the DOS Lemmings to enforce the one-skill-per-lemming restriction. I'd imagine it wouldn't be too hard to mod Lemmix to do the same, especially with it being open-source.

Not quite builderless, but attempting to solve Danger 29 (Pipeline Problem) with only one builder is a LOT harder than any of the builderless levels; To The End included.

Also worth noting that the most recent version of LPDOS removes at least one of the levels mentioned in your list (Phase Rotation Syndrome), possibly more. (EDIT: Also removed is "The Lonely Pole". In both cases, the new levels replacing them are not builderless.)

Yes, somewhat tricky!

[sidebar: More glory to you if you manage that, but it's not essential.]

As far as I'm aware, all versions have only the 3 tracks.

I understood the game surprisingly well. However, I did once while stuck on Mayhem 19, try messing around with codes and accidentally got the one for the next level... I swear there were also some other weird glitches that happened with it, but considering how long ago that was probably just me thinking the next few levels were really weird. I did eventually beat it, and it was Mayhem 29 (of course) that I got stuck on. By getting a code for level 30 (I looked this one up, no lucky guesses this time), I managed to beat it. It wasn't until about 10 years later I finally managed to beat Save Me. (I had long since beaten it on Master System, which doesn't have Save Me, by then.)

ONML, I didn't really properly play until about 5-6 years ago. I beat the majority of it quite quickly (within a week) once I started, but one level in Havoc (Lemming About Town) had me for a good few weeks... >_> (EDIT: Re-reading this post now, I should note that I had no trouble working out how to solve it, but there was some positioning issue or something in the Windows version that made it really hard to actually execute.)

Lemmings 3D, I somehow got the idea that you're meant to use virtual lemming mode... so naturally, I'm now used to playing it almost entirely with virtual lemming on. In terms of levels, Chocolate Drop was one I found surprisingly complicated, as was Fun Fair. I eventually beat Chocolate Drop but had to get a code for the next level after Fun Fair (again, this level remained unbeaten long past me beating the rest). I have however never managed to find the intended solution to Final Maze (mostly because I'm not a huge fan of the "work out where stuff sends you" type levels), instead going for a nice backroute I found very quickly. I actually found a lot of backroutes throughout L3D, unlike in the original where my solutions generally match up to what most other people find. (Spaghetti Junction and Tower of Lemlab are two levels I loved backrouting especially, although I did find the intended solution to both).

For those interested in the Master System version's unique musics (also the ones I used in Lemmings Plus DOS Project),

13 is original as far as I'm aware
14 is Miniature Overture
15 is Scotland the Brave
17 is Don't Dilly-Dally On The Way

Some levels though, depend on the time limit in other ways. For example, my level "No Time To Die" (from Lemmings Plus DOS), the focus is on speeding up the solution to meet the tight time limit. The basic path is obvious, but having to speed it up majorly complicates the solution, it's far more than just a matter of refining every tiny detail.

Any chance of a reupload?