Is it necessary to "dry" MEK with epsom salts before using it to purify mescaline acetate? The MEK that is sold at the local hardware store says "100% pure", but I was not sure if that accounted for a small amount of water.

Nope. Just stick the can in the fridge first. My friend tells me for best results before siphoning off dirty MEK stick the beaker in the freezer for 30 minutes and siphon it while its still very cold.