I'm not sure if I'm seeing it exactly but it sounds like you might need a bigger hole for downstairs. Maybe use a hole saw so you have more room to fish and can get the cable through looped if you can grab it.
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I'm mainly worried about damaging the joist that runs along the top of the lower wall. If I drill up from the receptacle cutout with the flex auger it will maybe go behind the joist but if they are doubled it will just go through one, probably at a weird angle.
The upper wall puzzles me a bit because it is offset and seemingly just sitting on the plywood then air, but I've been looking for the endoscope I could have sworn I accidentally stole from work to get a better idea of whats going on. I might just take down more drywall if I can't find it.
If you don't have wallpaper- I'd cut some more holes with a hole saw or even just a knife to see whats going on in there from different angles, easy enough to patch them back up.