EEngr: The problem is; these battery failures (fire or not) don't directly affect ETOPS range. The batteries are either there or not when they are needed.
When is either battery essential?
Loss of both engines has to be "impossible" for ETOPS; is it not equally "impossible" that power from at least one generator [on one remaining engine] will not be available?
Failing that, can the APU and RAT individually supply essential power for the ETOPS period? Can the RAT be deployed at cruise? At what airspeed does the RAT become useless? - that would seem to be the moment you need a battery. Too bad Boeing's math showed it was a waste of money to add a redundancy path between the two identical batteries.
It's amusing that the Boeing CEO and the head of FAA are solving the problem. IIRC the FAA grounding order directed that Boeing engineers satisfy their Renton-area office.
It will be even more amusing (though unlikely) if another jurisdiction took a less political view of the matter. Japan probably has similar commercial pressures given that their airlines are heavily invested, but the EU might have different ideas.