If you're wanting to use a fixed version of a level, you don't need to use the Lemmix editor. Just turn on LookForLVLFiles in the INI then place the level file in the folder named to match. I forget which name corresponds to which number in RotL, but if we were using the original game as an example, to replace Mayhem 20 you'd name it 0420.lvl
That being said, I'm working on a much-improved version of Lemmix (for general existance - my aims with this are a bit different to EricLang's; his seems to be extending the formats and games supported by Lemmix, whereas mine is on keeping as close as possible to the traditional Lemmings experience but without the DOS limitations and glitches), which I'll then build a more finalised version of this pack based on. So far I've got pixel-perfect trigger areas and object/steel positions, and I'm part-way through making indestructible steel (so far I've done the groundwork and fixed bombers, still have to do the other destructive skills), I've also fixed the sliding glitch.
I'm basing this updated player off the LPII code, simply because I saw no reason not to (it'll allow the use of LPII gimmicks with the released player that way - if no one ever uses them, oh well, no harm in them being there, though I'm sure we'll at least see a few interesting Karoshi gimmick levels). Though I can easily revert the bombers to the traditional ones, and apart from that (and a few other glitch fixes, which are just a start to what the whole aim is here), all the new features in LPII are extras that if someone doesn't want to use, they can simply not use.
And there's no intention at this stage to use this improved Lemmix as a base for an LPIII, though I'm not ruling out the possibility of it ever happening. Though I might make a custom engine if I decide to make a new Lemmings Plus...
Here's a shot of a slightly modified LPII level, with this indestructible steel in effect; the first one also shows pixel-perfect placement of objects and steel areas. It shows it for bombers, miners and diggers - still got to iron out some glitches with the bashers.