So the final hiccup was I had snipped the JST connector off the other battery wires. So for the right hand I just stripped the battery wires and soldered them directly into the through holes on the PCB. With that done, I was able to charge over USB and then both halves started powering on successfully with battery power, pairing to each other, and typing correctly to the host PC!

So the project is essentially done and just in the knick of time too because I have already left for a winter road trip and won't have access to my electronics workshop for several months.

So hopefully once I've settled into a longer stop on this road trip, I'll flash it with my layout and start practicing. Still undecided whether I should flash it with dvorak and try to start using it ASAP or flash it with engram and use it to continue learning that layout. I'm not ready to try that as daily driver yet as I'm only about 19 letters in to the keybr gradual addition of letters process, and even then my speeds are ballpack 30 WPM with poor accuracy.