Q1: not quite sure. Obviously monitor it. If the snake starts corkscrewing or stargazing, vet asap! It could just be something he does on his own. Mine spooked me a couple times with a tremble while she eats. Otherwise, fine.
Q2: this is part of why a good quarantine is important!
You are going to have to treat every animal in that room (possibly your whole household) as infested. Any bedding that was exposed (even unused) I would just toss. Start fresh in 3 weeks.
The lifecycle of a mite is about 21 days. Depending on how long you had this snake around the others, you may have moved mites all over on yourself or on items from the rack. Right now you are killing off the adult mites on the snake, but there can be eggs or larva in so many places...they may still cause issues for the next 2-3 weeks.
If you have animals in another room that still seem okay, a shower and fresh change of clothes before working in there is not a bad idea. Mites can be picked up on clothes and dropped in other enclosures.
I would take every animal in that rack and put them on paper towels for mite monitoring. A quick PAM treatment for them wouldn't hurt.
While you are changing out the bedding, make sure you are wiping down every surface on that rack that you can. Mites can travel pretty far just on thier own.
The snake that you are actively treating should be moved as far as possible to avoid further risk of transferring mites. A weekly mite treatment protocol will help get things under control since the next batch of eggs can hatch anywhere from 1-4 days after being laid.
I've never had to worry about mites myself at home, but I've used bleach and frontline spray to treat for them at my job.
Maybe some more advice will show up soon on the bedding questions and such.