So the ebay seller sent a second battery that demonstrated the same no-charge issue. I decided destroying a $12 battery (and maybe the reader) was worth the risk since my PRS950 was near enough to useless without a battery. Took apart both batteries (meaning I heated them with a hair drier and peeled the stickers off). These 2 photos are the "control boards", hoping that someone with more electronics knowledge than me can point out the difference...
Photos were WAY huge here, so a link instead (sucks, sorry):
https://imgur.com/a/2pjgjJz
I need to tidy everything up, but I have verified that it powers on without error to charging (on and standby). I'll have retained all of the "safety" that Sony implemented, just with a fresh battery pack. Going to keep the OEM for awhile, not sure what it'd be needed for, though aftermarket batteries are sometimes worse in "new" condition than worn OEM's.
None of this is hard, what you need mostly is courage to ruin something. Beyond that it's a glasses screwdriver, something to pry with (butter knife worked good on the battery), X-acto/utility knife (I used it to cut the leads/break the spot welds), solder and soldering iron. Flux would be nice too, but I used rosin-core (flux in the middle) solder and it's good enough. Took me less than 30 minutes, and hopefully I'll have more than an hour or two of reading charge.