So NTSB have begun looking beyond the battery, to other possible failure modes.
Boeing replaced as many as one hundred fifty batteries prior to grounding.
Almost enough to upgrade the entire fleet in service.
Until two old batteries lit up.
There has not been a "mystery" from the outset.
"it is inconceivable Boeing would not know what the problem was, since before evidence of it became known".
QED
Occam is being rudely abused, here and elsewhere....