But I also have this theory (totally not from spoilers--my own crazy theory) that Sammy isn't the YED's soldier, he's something elses secret weapon against the YED, which the demon is trying to bend to his purposes.

This is very clever -- it makes quite a lot of sense. I do think that Sam is a weapon, one way or another, and whether or not he's used by YED or against YED is very much up in the air.