I use an MES 40. Always fat side down. Meat is not a sponge, the fat just rolls down the meat, in the process keeps a bark from forming, and it's difficult enough to get a nice bark in an electric smoker.
You are over thinking it.. a pork butt is very forgiving
Make a decision and stick to it. I bet it comes out great
Smoke til 160 wrap add 1/2cup apple juice put back on till 205. Take off let it rest. Then call me.. I will be right over!!