Perhaps we're getting too hung up about the fire. Airport firies and NTSB say there was a fire, and my Gran said there’s no smoke without fire; but Boeing claims there wasn’t one. Who do we believe, who has most credibility?
It may not matter too much because quite apart from fire all Special Certification Conditions must be met. One requires “Safe cell temperatures and pressures must be maintained during any foreseeable charging or discharging condition and during any failure of the charging or battery monitoring system not shown to be extremely remote”. Quantitatively that’s a target of once in 10 million flight hours, whereas real life had 2 failures in 52,000 hours. Modified battery system certainly looks much safer but Boeing must convince skeptics that it’s at least 200 times more reliable. That's challenging.