Huh, I'm actually quite confused now how Clam managed to play the levels in Original Lemmings mode and be able to open the levelset in Original, ONML and CustLemm.
The way it is supposed to work is that in the LemmixStyles.ini file where you define the various styles, for each style you explicitly point it to a directory on where the files are located for that specific style, plus there are also subentries in the INI file where you explicitly map a graphics set to the corresponding GroundXo/VgagrX files (and potentially vgaspecX files). Thus by default it wouldn't be using any of the graphics files from the savelem folder unless you defined a style to do so.
Anyway, based on this description, here's a quick hacky remedy:
1) Make a copy of Lemmix.exe, LemmixStyles.ini and Lemmix.ini, and put them in your "savelem" folder.
2) edit the copy of LemmixStyles.ini: find the section (like the first one) that looks something like this:
[style_0]
StyleClass=TDosOrigStyle
CommonPath=c:\lemmings\
MainDataFile=main.dat
OddTableFile=ODDTABLE.DAT
StyleDescription=Dos Original Lemmings
StyleName=DosOrig
And change the CommonPath to point to the path where you have your savelem folder (eg. c:\savelem\) Optionally change the
StyleName and StyleDescription to whatever.
[edit: do NOT change StyleName, leave it as DosOrig or whatever it was!] Save changes.
3) Now open Lemmix with the copy of Lemmix.exe in your savelem folder, open the levelset, and switch to "Original Lemmings" mode [edit: or whatever you renamed it to; likely the editor will auto-select the right style and you won't even need to change that]. Clam's replay should work now. Note that we've effectively change the "Original Lemmings" style for this particular copy of Lemmix to point to the Covox Lemmings files, so you won't be able to use this style for example for levels/levelsets that actually call for the true "Original Lemmings" style. For those, go back to your usual everyday copy of Lemmix.exe with the unaltered style information for Original Lemmings.