First off good job on helping out a homeless snake. What kind of enclosure do you have the animal in? It's very important to get husbandry standards up to basic requirements before we begin treating mites. Aim for a 90 degree hot spot, ~80 degree cool side and ambients temperatures at least in the mid to high 70s. When dealing with mites you will want to only be using some type of paper substrate like paper towel, newspaper or butchers paper. Cypress mulch and any other shredded substrate gives mites the opportunity to hide and breed.

As for mite killer, I really like Natural Chemistry Reptile Spray. It is a non-toxic mixture that can be applied directly to the snake, the enclosure and substrate. Do not spray it onto the snake. Rather take a cloth, soak it in mite spray, and let the snake slither through the cloth multiple times. Before you do this I would soak the snake in a warm bath, with just a tiny bit of unscented dish soap. When I say a small amount, I mean a miniscule amount. It just needs to be enough to break the surface tension of the water so that the mites drown. Soak the snake for 20-30 min. While doing this, spray down the entire enclosure, and let the spray dry. Remove snake, apply mite spray as described above and put the snake back in the enclosure. I would do this every two to three days until all mites disappear, and then keep going for one or two more applications to take into account the larval stages. You will also want to spray some of the area around the outside of the enclosure.

As for defensive behavior, this absolutely normal. This animal has obviously been in a horrific environment and living situation, and will definitely have some resounding effects from it. But with time, patience and frequent handling, there is no reason the snake cant calm down. Yes there are some that might maintain an aggressive edge after years of abuse, but 'most' can become very tractable.