1. Development of an Atomic Bomb is only possible once you have reached the conclusion that a chain reaction is possible. AFAIK only the Allies every reached that conclusion before the dropping of the bombs at Hiroshima and Nagasaki. The German Nuclear program was looking at power generation not weapons manufacture, although it took the best part of an afternoon for the captured German Physicists to work out what had happened when they heard of the dropping of the Atomic bombs.
2. When discussing the use of Nuclear weapons at Hiroshima and Nagasaki, in this context, you have to do so from the perspective of Truman and his staff not on the basis of what history professors think (with the full benefit of primary Japanese sources and the benefit of hindsight). On that basis you have to conclude that there was no reason for Truman to expect an early Japanese surrender.
3. Casualty estimates for an allied invasion of Japan were in excess of 500,000. The US went so far as to manufacture this number of Purple Hearts (awarded for being wounded or killed in action by the US president), this stock of medals still has not been exhausted some 68 years later.
Given the expected level of losses and misgivings held by a number of US and British senior officers about Mac Arthur's invasion plans IMHO Truman's decision was the correct option at the time. Speculation about the Soviet dimension to the problem and the possibilities of the internal debate in Japan leading to a surrender while interesting while interesting do not, again IMHO, substantially change the decision process of Truman at the time.