All the research I've ever done has told me that NO wild snakes have mites. They have ticks, but not mites. Snake mites aren't even native to the area where ball pythons come from. I BELIEVE this is in the Barker's book, but I could be wrong.
There have been no conclusive studies on the nutritional value of live rodents versus f/t rodents. I believe they are the same as long as the rodent is consumed in a reasonable amount of time (ie: not sitting in the freezer for 2 years).
You are correct in that live does carry its own possible problems, but i believe the OP said she was feeding live because that was the only prey type those snakes took. Most bps will switch to F/T without a problem, but that is not true for every animal unfortunately. :/
OP:
As far as the mites go; if you want to continue using aspen, spray it first with PAM (prevent-a-mite) and then let it sit. This should kill the mites in the bag. Do this for every new bag you pick up and you shouldn't have a problem with the mites coming from that source.