Sorry to hear you are having so much trouble with the hidden level. The only other thing I can think of (if you have already followed the posted steps very carefully), is that you might just have too much junk in your MotS directory. Do you have all kinds of add-ons and mods on your system? If you have any other files related to the hidden level in your MotS subdirectories the game might not be loading the correct (.goo 'ed) version of the file). Another thing you can try (I haven't tried this) is to download my version of the level and use the patch commander to run it. I don't know if this will work but it might be worth a try.
Anyway, let us know if you have any luck.