Wouldn't doubt that. Had a few minutes to test things out this afternoon. If I go into BIOS settings after unplugging, I can see the SSD (and the eMMC) and everything is as it should be. If I reboot from the BIOS without letting it get to Windows, those settings all remain intact. If it gets as far as the Windows startup it all goes back to an unusable configuration and I have to power off / unplug before it works again.
Problem with that scenario is that the battery is used to retain the settings in the BIOS when the power is disconnected. If I replace or plug the battery back in, I never get the "reset" I need to be able to boot from the SSD. So battery can't go back in until I can figure out what's causing the problem. (It's not the battery... the battery just stops the BIOS from shaking the bad settings.) The BIOS keeps the settings as long as the power input is live, so I can still test the fix without having to crack open the case and disconnect the battery again if changes don't work.
Pertaining to your earlier note:
1. Didn't get a chance to look at drive format or letter, but this can't be the issue as things DO boot when the BIOS can find it.
...
4. Played with "CSM" settings. With CSM enabled, it sees the SSD but not the boot partition on the SSD. It will boot, but gets to a black screen with a cursor that never changes. With CSM disabled, I can see the eMMC (if enabled) but not the SSD (on the second or any subsequent reboots).
* edit - worth noting that CSM is [disabled] by default on that first boot when I can see the SSD and correct partition in the boot menu.
Corey