First off, I'm sorry to hear about your snake having health issues and now mites. I recently experienced a very frustrating battle with mites on one of my snakes. I tried both Provent-A-Mite (PAM), and a solution of Nix over a period of about 7 months and still the darn things came back. I completely cleaned and vacuumed the cage, washed it with a bleach solution, sprayed the "solution" on and waited sometimes 3 weeks before returning my snake to the cage. Every time, they came back within a week or so and believe me, it was VERY frustrating to say the least. This last time I tried the predatory mites and so far, so good
While you can certainly leave the snake in the cage with the predatory mites, I chose to removed her for 3 weeks to let the mite battles progress without a food source for the snake mites. It's now been about two weeks and she is still completely mite free. I've supplied a link to the company that I bought the mites from. Great people to do business with and very willing to answer any/all questions promptly. You will get enough "medium" with the mites and eggs in it to treat about 6 or 7 cages. I treated all of my cages, but I put an extra dose in the infected cage for good measure. So far, I am a very happy camper and hopefully mite free for a long, long time.
Stratiolaelaps scimitus (Hypoaspis miles) - Fungus Gnat, Thrips, Snake Mite Predator
Good luck and please keep us posted on your experience and progress.