Folks, very interesting development!
I pulled the valve body, split it, and found a potential issue with the "Basic Pressure Control Valve".

Here's a photo of the valve after I removed the spring:
https://ibb.co/PtwBjSc

Here's the location on a diagram:
https://ibb.co/B263VfZ

Ok stupid question... this thing is supposed to move right?! I assumed so, because it's a valve, but it's not moving AT ALL. I have gently pushed the "rim" visible in my first photo, as well as further inside and I cannot get any movement whatsoever.

Assuming I haven't thought of anything, is this my smoking gun? If this was jammed in such a way that it was venting working pressure, that would certainly do it..

I just want to make sure I'm thinking of everything here. If it is indeed stuck, I'm not sure how I would get it unstuck without marring anything.

TIA!