If you think the US only burned two Japanese cities, you should look at the history of the US's bombing campaign. Firebombs were used a lot.
As for "taking the head honchos to the tests", let me propose a few counterfactuals:
1. Japanese leadership sees bomb, changes air defense priorities to target small groups of planes and not mass attacks. Enola Gay is shot down. Invasion happens starting in October; 500k+ die at minimum.
2. Japanese leadership thinks the US is soft and lacks will to victory, holds out even after bombs are dropped. Invasion happens starting in October; 500k+ die at minimum.
3. Japanese realize this isn't really worse than the existing mass firebombs, distributes more production and population away from cities. Invasion happens starting in October; 500k+ die at minimum.
Considering how close the surrender seems to have been in our timeline (personal decision of the Emperor, overriding advisors and military leadership) I would not assume that a softer path would work.