I don't have experience with mites, but I'll chime in on two other things...a 95 degree hotspot is a too high, so I'd recommend lowering that to about 85-90. Is your UTH regulated by anything?? If not, that will help with the hot spot control...UTH are notorious for having huge fluctuations in temperature and can easily heat to well over 100 degrees and burn your snake

Next, get a gram scale so you can monitor his weight. They are inexpensive and easily found on Amazon. In terms of the feeding, a pinky is much too small. BPs come out of the egg already big enough to eat mouse fuzzies. At 18 inches, he is likely ready for a mouse hopper at least. Everyone on this site will tell you to feed a prey item that is 10-15% of your snake's weight (hence the usefulness of the gram scale!) and no bigger than the widest part of the snake. Once you can determine this, you'll be in a better position to offer your snake food that he may be more interested in. I think while he's going through all the stress, assist feeding is not a good idea. This is a last resort option and he is not likely to eat due to all the changes.

So, get the mites taken care of and work on getting him more appropriately-sized food once he's had some time to settle, like you said. And humidity should never be lower than 20%!! Keep it around 50%...that doesn't sound like much of a reptile expert to me!