To follow up on this, I finally got all the parts together (some were seriously delayed thanks to chip production issues), got the whole thing to my bike shop, and have the bike back. I'm trying to figure out exactly how they mounted the battery in the downtube (the mount holes are unoccupied, so it looks like they just shoved the battery in? That conversation is ongoing and I'm not sure if they're wrong or I'm wrong), but after looking at how the Junction A is set up, I have to say I'm a fan. It slides on to the bento box pretty securely, but it slides off easily to charge it, too. I honestly didn't realize how clean it would be once they installed it.

The holes it slides across to stay locked in:

And how it looks installed: