I searched a bit, but I think I was being a bit too specific in what I was searching for. I'd run across the kirich blog link previously when I was trying to determine what would be needed to make my own load, then couldn't find the link again later... (thanks for pointing me back to it).
After looking at those schematics and reconsidering if CV might be PWM and I made a mistake, I determined that my assumed good DL24M-c is faulty as well:
I hooked the main controller and expansion together with only VCC, Fan, GND & CV, then hook up a battery to each unit's S & D; the expansion unit didn't pull any current while the main one did. I reached out to Atorch via AliExpress and yet again, they refuse to do anything about it. (so 2x bad expansions from separate orders 3 weeks apart).
I also just found this (coincidently while looking for pictures), which is pretty much exactly what I was seeking.
https://flaviutamas.com/2022/dl24m-electronicshttps://flaviutamas.com/2022/dl24m-reversing