So, if I have understood this correctly, Boeing are going to basically say "The batteries are going to spontaneously catch fire, but its OK because we can put them in a box".
Here is a test for you Boeing engineers and execs - build your boxes, install them into a couple of 787's, then load yourselves, your wives / husbands, your children and grandchildren. Take the planes up to normal cruising altitude, deliberately set fire to one (or both) of the batteries and then carry on flying for the rated ETOPS time.
If you survive, do it again. And then again and again. Keep doing it until you have come up with a proper solution.
Oh, and if the FAA approve it, load all the FAA execs & their families in there too.