Working backward, FAA accepted a single cell would not cause propagation (further runaway). So the assumption is BOEING designed, built, and tested a platform that demonstrated this to the satisfaction of BOEING. And to FAA.
The battery failed the real world test.
So working
backwards, Boeing did not test it for certification, unless they deliberately introduced a battery with known design flaws and covered it up.
2) The problem to isolate a defective cell is: You need a costly relay and you reduce the voltage of the battery. Itīs not practical.
Sorry, thats bull****. The individual cells are monitored and isolated if defective, they have to be. A relay costs a few dollars in a $16k battery FFS.