I'm going to call this one a Work In Progress, although I may not actually fix it.
Honestly I've tried several methods (within my limited modeling experience) to fix the shoulder. The model looks fine in Blender, but Cura refuses (I mean absolutely refuses) to slice the shoulder properly. I even dropped a Sphere mesh into the shoulder to try and fix it, but again, Cura ignores it.

You can see what I mean here: https://imgur.com/0sz4xY0

If any of you want to fiddle with it, go for it. In the meantime, maybe I'll get a couple zombies on my table before game day! :D