The only thing you are losing as the mice become more freezer burnt is nutritive value and palatability (or whatever the equivalent is in snakes - scentability?). They do not become harmful to the snake. White spots on the skin aren't going to harm the snake, it's just dehydrated skin. If the desiccation penetrates into the muscle, etc., the nutriative value does down, but doesn't hurt the snake.
To avoid desiccation, the gold standard is vacuum packing. Apart from that, I try to slow it down by always keeping them in sealed plastic bags with as much of the air as possible expressed out. Leaving bags full of air in with the pinkies will accelerate freezer burn. Use as small a bag as possible and express the air. Double bagging helps, as does wrapping in aluminum foil or sealing in a tupperware container.
As far as when things may not be so good for your snake, if you thaw out a mouse and it smells foul...it is foul, chuck it in the toilet. If it's discolored, mushy, or the skin is so thin that it ruptures...I chuck them. If they are dark purple instead of pink or whitish, I chuck them.
That's my practice with them, anyway. Good luck. Enjoy the swap this weekend.