Seems a little clearer to me now.
Boeing's initial tests concluded that a single cell overheat wouldn't propagate to the rest of the battery, that's obviously not the case as it's been determined that cell # 6 had a fault which took out the whole battery.
Adding ceramic spacers to give more separation to cells sounds like a plan, however prevention is better than cure and better QC/specification on the cells before they get installed would be good.
Even then, the MTBF on any cell should be at least an order of magnitude higher.
One thing, I don't get is how a standby/back-up battery is on what appears to be a common bus, if there's an electrical problem I'd want a clean, uncompromised power supply on a previously isolated bus.