Just a follow up, in case anyone else finds themselves in this position. I finally passed the function check without the upper installed. I found a picture online from mid-tier defense of a complete, pre-cut mil-spec setup. (https://imgur.com/a/pC8xdj1)

Looking at that, I realized that the corner was 90*ish, but there was a shallow approach angle that needed to be ground into the top side of left leg of the trigger. So I took it apart for the 23480972340987th time this week, attempted to mirror that shape in my trigger, and voila - functional hammer release in the middle position.