Another thing to check is the driver control panel, there's always one. There may be non-obvious options about how you want the input channels to be treated by the host OS. As an example, IIRC, in MOTU gear, you can actually get surround sound by checking a box.
In my specific case with my Audient, I get stereo pairs in the host OS (outside of an ASIO context) for both inputs and outputs.
Well, I fixed it, but unfortunately not through Windows sound settings. The driver control panel for the Zoom mixer exposes two options (sample rate and buffer size) and only buffer size can be changed lol
I went back to the ASIO4ALL and OBS-ASIO route I'd tried originally, re-installed the drivers for the Zoom, enabled pull mode via ASIO4ALL (not sure if that's still checked but I was pressing buttons at this point and it helped). Then after looking everything over and making sure I was getting the sound from the master output via USB, I disabled duplication of sources in the transitions menu in OBS. It occurred to me a few days ago that that might have been the original source of my problems the first time I tried this. I just had to set my ASIO input as a source in every scene, but then I was also able to enable audio monitoring via my laptops headphone port, and I'm getting quality, digital, next-to-zero latency audio.
Even though this thread itself didn't quite fix my issues it made me revisit the issue with new ideas, and I definitely appreciate the time and effort