I think the discussion of whether battery failure affects ETOPS has got a bit over-technical. The problem is not that the battery fails, but that it fails in such a way that it might start a fire in a part of the aeroplane not normally containing fires. That, surely, has some influence on how far away you get from a diversion.
I have no technical knowledge, but I live in Auckland, so ETOPS is a matter of existential concern to me. I even had to reassure myself the first time I flew in a 777 from AKL to LAX. Paxen like me choose airlines, sometimes based on the equipment they use, and although I like Air NZ, I'd be reluctant to fly with them on a 787 unless I was sure this problem was really, thoroughly, fixed. As I say, I have no professional knowledge, but I'm reporting in from the commercial reality.