Just to be clear on B787 battery issue
GS Yuasa self-certified the battery
JCAB accepted GS Yuasa self certification
FAA accepted JCAB certification
EASA accepted FAA certification
Boeing just framed the certificate and hung on the wall
So Boeing, FAA and EASA had no real involvement in testing and certification. But they were deeply involved in steel box design and certification.
There is no permanent fix (without steel box) to B787 battery because GS Yuasa never accepted there is a problem with the battery in the first place.