I figured it out this morning, and like I thought, it was kinda dumb. Of me.
THIS is the solution for the raw audio coming through the headphones:
See how Input 3 - the Hi-Z, because I was working with something with active pickups - is set to 0? THAT turned off the direct monitoring. I was thinking of the board as routing and it’s … well, it’s routing, but I was looking at that input and thinking that the control affected the signal being passed through (as it does in Cubase), and it doesn’t. It affects the levels being passed IN THE A/D. Turn that off, bingo, no direct monitoring.
So like I thought in the first place, all my problems were based in ignorance and not knowing where to look - the inputs are kinda offset from what I expected, so I need to set the exact right input instead of “input 1” because there’s a translation with my audio device in Cubase/MacOS (it’s a combination of things, it’s not like ONE ASPECT of the entire system created all the confusion), and I needed to understand that the levels in dspMixFx are effectively loopback levels in the A/D and not levels for the signal.
There’re still things to fix in my setup, but I think I’ve figured out what I needed to for the most part.
Thanks to all who looked and helped!