I don't think placement could solve the points you mention.

Sure, debris can damage the underside and cause the car to catch fire, but putting the packs in the front or rear of the car can cause it to explode on impact as well. Having it on the underside will probably cause fewer fires than having it in the front for example.

That said, consider gasoline cars: The fuel needs to travel from the tank to the engine. Effectively, an impact anywhere on the fuel line can cause a fire.