I'm with Jennifer on the probable stress diagnosis. IF there are no other underlying health issues (which a herp vet needs to check for) I say leave it alone for two weeks--totally alone--and then try and feed it a live crawler rat or hopper mouse at night. Bump the warm end temps up ASAP to the low 90's, and keep all the hides and water dishes you have set up.
Take what the pet store employee told you with a BAG of salt (not just a grain!).
I guess if the mites are gone, and your snake didn't react negatively to the treatment, good enough. I would be leery of using a treatment that wasn't intended for snakes, however.
You may want to lower the humidity a tad, to around 50%-60%.
Is the incandescent lamp on 24/7? If so, that could cause significant stress.
Good luck, you are doing ok so far! Go back to the pet store and punch that ignoramus in the nose! (Just kidding!)