If America dropped atomic bombs in Tokyo,and Hirohito and responsible people were killed by them, Who'd decide Japan should surrender?

Answer 1

The Emperor wasn't killed and he later gave the speech surrendering. He sent his then Minister for Foreign Affairs Mamoru Shigemitsu to sign the formal documents aboard the USS Missouri. 

Answer 2

It was either God or the Japanese themselves.

Answer 3

If the US were being bombed constantly by an enemy force, do you imagine the president would be sitting in Washington, watching the world burn from the Oval Office?

 

During times of war, when the enemy is at your step, leaders naturally delegate from positions of safety and security.  And so those in power would of been nowhere near the proximate of such attacks.