Let's operate off of the assumption that not deploying nuclear weapons against Imperial Japan at the end of WWII would have necessitated either an amphibious invasion or a long and costly blockade/conventional bombing campaign to force their surrender, which I understand to be the assumption that most academics that study that period share.
Presuming that the Allied forces opt for an amphibious invasion, let's disregard the greater geopolitical ramifications that such an operation would cause. Instead, I'm more interested as to the effect that an amphibious invasion likely several scales larger than Overlord would have on the nation of Japan.
Would it be a combined American and Soviet invasion, or just American? If the former, would we see the islands split in to a la Vietnam, Germany, or Korea?
How harshly would the Allied forces punish Japan after a likely bloody victory by amphibious invasion? Would they completely demolish the Imperial system, and force pure Republicanism on the Japanese?
What kind of environmental destruction could be anticipated? Would the entire Japanese countryside - that OTL provides picturesque scenes of centuries old traditional Japanese houses - be charred rubble by the end? Would Hokkaido manage to go untouched, or would there be some sort of grand, Republic of Ezo-style last stand on that (relatively) isolated island? Would major cities like Tokyo, Osaka, and Kyoto end up in a similar state to Hiroshima and Nagasaki in OTL, just completely flattened and burned to the ground - more than those former cities were in OTL, anyways?
How many civilian casualties can we expect? I've heard about plans for mass suicides and kamikaze/banzai charges by civilians, but I don't know how grounded in reality those are. My understanding of pre-WWII Japanese population figures is that they were around roughly one hundred million. How low could we see that drop after an amphibious invasion? Can we expect Soviet Union-tier civilian casualties, with 10%+ of the pre-WWII population being killed? Maybe more? 20%+, even?
OTL, occupying American forces managed to prevent large-scale war crimes in Japan (though there were, obviously, plenty of incidences on a smaller scale) but could we see more incidences of human rights abuses by American and possibly Soviet soldiers on the Japanese civilians, especially if the latter put up significant resistance to the invaders, a la the Vietnamese in the Indochina Wars?
Speaking of resistance, how long/to what degree could we see a resistance movement after the initial invasion is said and done with? Presuming that there are plenty of Japanese soldiers and militants pragmatic enough to not charge gloriously into American/Soviet machine gun fire, could we see a Japanese resistance movement against Allied occupation and a future Allied-controlled government for years to come, a la the plans for the German Werwolf program?
Thanks for any opinions or ideas you have to offer!