Seems like 6 hours is the magic number. If the battery is left unplugged from the mobo for 6 hours, (I guess it's discharging) plugging it back in, followed by plugging it back into the wall charger, then immediately pressing power, it boots up. I've done this 4 times now. The bootloader must be jacked up. 4 times, because I've tried various remedies to fix the bootloader, or get into custom recovery.
End result, the tablet runs, I just can reboot it anymore. That's okay I guess, just ordered a Shield tablet.