Your snake can wait until the PAM order arrives with no significant ill effects, unless it is a really bad mite infestation. From your statement that you don't see any on the snake, just in the water, I assume this is not the case, so you should be fine.

The mites apparently are uncomfortable for the snake, so some people choose to buy one of the mite remedies that you should be able to find at a local pet store. These will kill live mites (but not the eggs), and make your snake more comfortable in the meantime.

Snake mites do not transfer to people. You might find one crawling on you or your clothing, but you won't get infested with them. There is the possibility of them hitchhiking and then transferring to any other snake you own, or if you visit someone else who owns a snake, so you should be careful with your QT procedures until the PAM has had a chance to do its job.

I'm not sure about the water temp question. If the ambient temp in the enclosure is no lower than 80, I would assume the water temp also is no lower than 80, but I'm not sure about that. If you are concerned, move the water closer to the heat source temporarily, and keep an eye on the humidity and make sure it doesn't go too high.