Ok, I'll try to answer them all:
1. no, it is not harmful to your snakes, you can bathe them in it. Don't spray it directly into their mouths though.
2. No, you can leave all the feeders there, it's not like those harsh chemical mite removers where everything within a 100 foot radius dies (Provent-a-Mite). As long as you're just cleaning out enclosures, you'll have no problems.
3. Bathing daily is overkill. Completely clean the enclosure(s) that are infested twice a week, and bathe your snake the same. Everything should be mite-free within two weeks or even sooner.
4. Stick to either unprinted newspaper stock or paper towels. That way, you can see mites better if they are still looming. Also, be sure to change the water dish daily.
5. Stick to the same temps you have now. Mite eggs will hatch in a variety of temps, but you're going to KILL THEM ALL with your Nix treatment, so it's not a concern.
Good luck, and let us know how it turns out.