Should they have hung Hirohito?

"In my opinion, the A-bombing of Japan was a necessary evil to end WW2. It is known for fact that the US planned to invade mainland Japan. It is also known for fact that the Japanese were determined to defend their mainland islands without any regard to casualties."

You are arguing that they surrendered because of regard to casualties-ie 2 bombs, this sort of undermines your claim that they would defend without regard to casualties, don't you think?

..........
" Had the invasion taken place there would have been many millions of casualties, predominantly Japanese (the Americans estimated about one million of their own). The two A-bombs took the lives of less than half a million people and the war was over. This must sound like the devil's arithmetics, but the facts are undisputable."

These facts have been frequently disputed. It seems to be a marker of self doubt that the pro bomb case has to vehemently stick by the worst case scenario for invasion, and the highest casualty estimates, and to ignore the aggressive, rapid and successful soviet entry into the war against japan. When weighing arguments it's a good idea to keep your thumb off the scales.
..............................................................

"Wisely afterwards, the Americans ruled Japan through Emperor Hirohito, him being the unifying symbol of Japan, and through him making the Japanese people accept a constitutional monarchy."

Survival of the emperor and the imperial house was an express term the japanese sought in their peace overtures. Peace could have been had just about anytime from feb45 had the US explicitly accepted this term which was ultimately granted! I know it's not central to the bomb argument but Hirohito's criminality , and that of numerous members of the royal household was beyond doubt. Hirohito should've hung.