Another aspect not covered in the OP is the efficiency of US nukes. Much/most of the damage done was not from the bombs. If you measure damage through the heat energy released, most came from the burning of the cities rather from the fission in the bomb itself. The firebombing of a city releases as much energy as a nuclear explosion, doing a comparable amount of damage. At the time, a firebombing raid required fewer resources than a nuke. So, from an energy standpoint, the nukes were not an economical means of destroying Japanese cities.
Why then were they used? Why were they used in the pacific and not in europe? (Insert massive screaming rants here.)