This shipment of 3D printed parts arrived, and I was able to assemble them together and boot the phone off of batteries in the back compartment that were loosely connected by some wires in the spacer component that I pushed into place with my hands. I had the wires floating around in some box of wires from an electrical engineering college course from a decade ago, and did not know if they were necessarily correct for the job. But I used them anyway, and tried to bend them into place on the device.
I found that when the interior battery on the Librem 5’s main compartment had been removed, I was able to still boot the device from the second battery above that compartment with the removable door, however the Librem 5 was refusing to boot with only the one battery below the camera inserted which would have presumably meant the loosely added wires were not quite touching that battery, or not until the second battery was added. [Because of this, I have also not confirmed or denied whether the second battery is able to join the circuit in parallel and trigger longer battery life.]
The batteries seemed to report strangely low charges in the range of 10%-20% by the time I finished fighting the wires into place, even though they had originally been fully charged to my knowledge when I started. Does that mean a bunch of shorting was happening?
It also seemed very difficult to shove a custom wire into place and have the battery boot off of this reliably, so one of the things I did was to wrap the wire around the springy metal connectors that usually touch the battery to hold it in place. After some testing, when I removed the battery house, this effort to wrap the wire tightly around the metal connectors resulted in one of the metal connectors ripping out of the Librem 5 and I have not been able to finely manipulate it to go back into the Librem 5 (for example if I wanted to change back to using only 1 battery) which seems highly concerning.
Edit:
No attempt was made to charge the phone while multiple batteries were connected via the shoddy wiring, even though a “shoddy wiring” boot has been proven to be possible and it showed strangely low charge on the battery which remained after removing the battery and testing them alone in a standard Librem 5 (suggesting that either shorting nuked the battery or else I started with batteries that were not as well charged up as I had expected).