I'd say Floaters is your best bet. That's if you can't find some other small thing to place it under another department. Bad Het, et cetera. (The one I was thinking about would probably be Bad Het.)

Something very general, perhaps to which everything else is sort of a sub-set?

Or floaters. Floaters tend to go by continuum rather than subject, so they're always an option.

If there's any romance at all and it's not slash, it could still fall into the Department of Bad Het. Then there's the Department of Technical Errors if it's full of spelling mistakes. Do we have a Department of Characterisation, or something along those lines? When in doubt, call on the Floaters.

I think it depends on your interpretation. If the characters are so OOC that they may have been replaced by other people, and these other people seem Mary Sues (or at least one of them), then the DMS could still handle it. It's a case of character replacement. Look for plotholes where the canons may be hiding and then charge and kill the Sues.

Oh, and floaters seem to handle pretty much anything.