fire is only acceptable against risk up to some probability of occurance
No, there is no risk here so it is pointless to talk about "probability". There is only probability of inconvenience to the airline if the battery goes bust in mid flight - at destination aircraft will have to be serviced, most likely the next flight will be cancelled. As to probability - it can be approximated - there was another occurrence after the "fix" so the battery fails about once per ... say 10,000 flights but then other components also do fail and more often than that. And by the way, there is no "energetic fire", there is hardly any fire since the steel box also cuts the battery from oxygen supply. And this is exactly what happened when it failed last time - the battery with the steel box were swapped.
As I recall, there was at least 1 engineer (who was sacked by Boeing)
Who gives a **** about 1 engineer, there could be even 100 engineers, the extent of testing is strictly determined and controlled by the certification agency.