In the short run, I'd be more worried about the diarrhea and "leaking several times a day" than I would the mites.

Also, if you post a pic of that lump you mentioned, someone might be able to identify it for you.

PAM is an excellent product for mites. That stands for Provent-a-mite; it is not the cooking spray. As far as where the mites came from, it is likely your snake had mite eggs on it, and you are just now noticing them because they just hatched.