Ok, after some thought, here's my current, totally speculative theory on the "falling distance saga":
It could be the case that, the ONML we download from abandonia.com is actually a hacked copy of the demo version of ONML. (Does that exist?)
Since the demo version of ONML does not have copy protection (I think), it would make for a perfect candidate to convert to a full version of ONML that lacks copy protection. I seem to remember hearing similar things having been done with the original Lemmings (ie. make the demo version of Lemmings run the full version's levels.)
Other possibilities include hacked copy of Xmas Lemmings, which I believe are also copy-protection free. The Xmas-sy graphics are probably stored in the .dat files rather than the program itself, so as long as you switch to the ONML .dat files you'll get the regular non-Xmas-sy graphics.
This would then explain the discrepency with the falling distance as reported by Mike and Shvegait. Since Mike said the change in falling distance occurs as a fudge to fix one of the ONML levels, it would probably occur late in the development cycle of the game, well after the ONML demo was released. And so the official, full version of ONML would have a higher falling distance than the demo version.
Anyway, that's just my current theory. The key evidence we need would be an official, from-the-floppy, unhacked, full version of ONML, if anybody has one. And test out what its falling distance is.
As for why the CD version of Lemmings 1 has a higher falling distance, as well as the Windows version. Well I'm not really sure at this point, but one strong possibility is that if they were made and released after DMA made the falling distance change in the source code during ONML development, then they too would get the higher falling distance, if nobody remembers (or deliberately decided against) to undo the change first. It could even explain why the CD version supposedly handles the music playing correctly. [Note: I haven't downloaded the CD version yet, I'll do it later tonight.]
CustLemm would then probably have come from either the CD version of DOS Lemmings, or the official full version of ONML.
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It still strike me as a bit incredible that DMA would choose to change something so fundamental as the falling distance (which, after all, potentially affects every other level) in order to fix a problem with a single level, instead of changing that level itself to accomodate.