While I agree with you analysis that the A-Bomb was unnecessary for Japanese surrender, I think it a bad choice to link to a David Irving article, the guys credibility couldn't be lower. I'm busy now but I'll see if I can't dig up later better links.
From how I understand it, the Japanese concern was with their Emperor. Prior American statements had made the Japanese think that the Emperor would have to go, when the Japanese learnt that they would be able to keep the Emperor in place they were willing to surrender. The A-bombing had little to do with the decision making process, Japan already knew they were defeated, if the Americans had spent more time learning about the culture of their enemies (a lesson unlearnt I fear) they could have got the surrender without the bomb.
I think this link describes the surrender decision
I think the bombs were dropped as a show of strength to Russia who had just entered the Pacific war