... Boeing encases the battery in a box and voila! It is magically not only safe, but also exempt from any requrement to actually, you know - work, since the box doesn't address it's reliability in any shape or form. ...
That's right, the box doesn't address the battery's reliability in any shape or form
There two separate problems.
One is the battery problem, and how to make it reliable.
The second is the enclosure, and how to prevent the battery from damaging the aircraft. The enclosure is not intended to contribute to the reliability of the battery. The other changes are intended to take care of the battery problem.
But now we have a third task; how to ensure that the stainless steel enclosure is itself manufactured, assembled, and maintained adequately.