Okay so I did finally get to an answer. Honestly, it's a bit hacky and I still don't get why it isn't changing my lockscreen BG like it should, but whatever...
Basically I started by opening the file navigator with root permissions:
sudo -H nautilus
then I navigated from the root directory to
/usr/share/backgrounds and from there the lockscreen background that GNOME seems to be stuck on is called
warty-final-ubuntu.png. I backed up that file, then copied the background I wanted and changed its name to that. It worked immediately.
Again, this feels stupid and hacky. With my lockscreen UI looking different from everyone else's, and the built in lockscreen background switcher not working in either the system settings nor the gnome-tweak tool, I feel like there is a deeper issue here that isn't being addressed. But nobody seems to really know, and this works for now I guess. So I'll just leave it at that.