SoS
I think you touch on the most important question... The FAA requires a BACK UP electrical supply, for a very short period should all else fail.
The Batteries have no chance of pressurizing the Hull, and the requirement is for five minutes of service, roughly enough time to get the humans down to breathable air from flight levels, in an emergency.
It is looking more and more like the 787 engineering scheme is to incorporate a back up system into a sustainable part of the in flight regime of systemic electrical supply. If so, they violate the purpose of the regulations from my point of view.
At the very least, and I pointed this out long ago, the Yuasa batteries should be round, wound without rectilinear stress on the plate, such that heat can be dissipated, rather than concentrated in the case.
For that matter, the case should be cylindrical as well, if only to honor the principle that square batteries create problems, if the plate is rolled.
So from the outset, the design appears to challenge best practice, why? Because square "looks" better?