1. Really, anything you buy has the potential to have mites in it. Personally I would ditch the cypress mulch. Once you get the problem cleared up, maybe you could try aspen (my personal fave).
2. I am pretty sure P-A-M will get rid of the problem completely but I would spray it once a month on cleaning day just to be safe. Not sure about them setting up shop, lol.
3. To help humidity the best thing you can do is ditch the heat lamps. They suck the humidity out of the tank like there is no tomorrow. If you haven't already you can try covering 80% of the screen top with plastic or aluminum foil. You could also try adding a little bit of sphagnum moss into the warm hide.

hope this helps!