3-days is a pretty short time for such a major shift in politics for starters. Would a 3rd or 4th have been justified in the 6 day gap before their August 15th surrender announcement if the US had had them available?

Also the justification for the second bomb, that it was required to demonstrate it wasn't a one off doesn't require the that bomb be dropped on a second city either. The main thrust isn't that the bombs weren't effective it's that the extreme nature of the act requires extreme justification bordering on it being the only option but for a bloody home island invasion. See my other long posts about the anti-moral-bomb argument in this thread, I don't want to duplicate them here to avoid spamming the thread.