Although it has been answered at other places in this thread, I will comment that I do use reptile relief as well, and wipe it directly on the snake as you did with no ill effects. I wish I had a better answer for what happened to your snake, but I don't. Lastly, while I will not argue with your vet over the use of Ivermectin for mites, I can say that PAM has gone through rigorous testing, and it is a great product to use for the eradication of mites, and has proven time and time again to be very successful when properly used.
Good luck with your snake,