The UnMirrorStuds.exe program generates stud-mirrored copies of the official parts alongside those official parts (in your LDraw directory). If you look there, you should see various .ums (and possibly .um2) files. And the .ums version of your original model generated by running the program should open in MLCad without any problem. I just tested with a relatively simple model (which only generated two .ums files in my LDraw parts directory, plus one .um2 in parts/s). MLCad opened the file without any errors. I did have to enter *.ums into the file name field of the file open dialog and hit enter in order to see the main file.
Note: to get rid of all of the *.ums and *.um2 files that UnMirrorStuds creates in the LDraw parts directories, run UnMirrorStuds with the -d option, and give it your original LDraw file. It will delete the .ums version of your LDraw file, plus all other .ums/.um2 files that it created when you ran it the first time.
The fact that the files have a different extension makes no difference to MLCad or LDView with respect to viewing the file. (I wouldn't advise trying to edit a .ums file in MLCad, though.) It shouldn't make any difference to other programs either, but I can't be 100% sure of that.
If you're willing to send me your LDraw file, I'll test it myself, but I don't see why it should behave differently in MLCad from the file I ran. Send the file to the LDView email address.
Note: to get rid of all of the *.ums and *.um2 files that UnMirrorStuds creates in the LDraw parts directories, run UnMirrorStuds with the -d option, and give it your original LDraw file. It will delete the .ums version of your LDraw file, plus all other .ums/.um2 files that it created when you ran it the first time.
The fact that the files have a different extension makes no difference to MLCad or LDView with respect to viewing the file. (I wouldn't advise trying to edit a .ums file in MLCad, though.) It shouldn't make any difference to other programs either, but I can't be 100% sure of that.
If you're willing to send me your LDraw file, I'll test it myself, but I don't see why it should behave differently in MLCad from the file I ran. Send the file to the LDView email address.