Hitler was elected once, but then took over as an authoritarian. I think that is fairly well-known.

So according your construction there is great uncertainty as to what would have happened had the atomic bomb not been used. I would disagree, but let's continue with this line of reasoning. You list invading Japan (making landfall) as one of the options. Presumably you listed this as an option because even you believe that this option had a reasonable chance, as opposed to a negligible chance, of being carried out or of being necessary at some point in the future. In that case, Truman could have still justified using the atomic bomb by observing that he preferred the certainty of ending the war with atomic weapons (at no cost in American lives--nobody is arguing that during wartime an enemy life is worth the same as American life) to the chance that an invasion of Japan was necessary. Truman had to contemplate questions such as "how many Japanese lives is it worth to me to be able to have say that there is a reasonable likelihood that I have saved perhaps a smaller number of American lives?"