Worst case, realistically, is based on the study William Shockley did for Henry Stimson. Fatalities directly from fighting were estimated at 5-10 million Japanese and 400,000-800,000 Americans, on top of casualties of 2 million and ~700,000 from the war to date. Call it a total 7 million Japanese and 1.25 million Americans.
Japanese planning called for cutting rations to the civilian population, accepting that this would mean allowing a significant proportion of civilians to starve, in order to maintain fitness of the fighting forces. I think the expected casualties were of the order of 20% of the population. Given 70 million prewar and deducting the above, that's another 12.6 million casualties, leaving 50.4 million Japanese at the time of their inevitable collapse in early 1946.
After all the fighting, with extensive use of nuclear, biological and chemical weapons, a famine in Japan is all but certain. IOTL, it took herculean efforts by the US military to avert one after the poor rice harvest in 1946 - ITTL, they've just bombed, poisoned and burnt what crops did grow, and destroyed the transportation infrastructure. And, after the viciousness of the fighting, it's unlikely the US will be in a mood to be as generous. Historically, casualties from a severe famine can run to about two-thirds of the population between starvation and disease. There go 33.6 million more.
After the fighting subsides and the famine (which will probably last several years) ends, the remaining 16-17 million Japanese will be disproportionately between the ages of about 3 and 18. Most of the military age population, including virtually all males, will have been killed fighting. There will be virtually no elderly people, the famine will have seen to that.
In short, Japan will be a depopulated agrarian wasteland with no cities, littered with unexploded ordnance, abandoned military equipment, and contaminated by radiation and CBW. The demographics will be a complete disaster. The surviving population will be scarred - PTSD will probably be near-universal, and there'll be a pervasive attitude of 'do whatever it takes to survive.
But the fighting and deprivation will have allowed the more depraved human instincts free rein. There may be war crimes trials for some of the more egregious violations of common human decency, but a lot of atrocities will go unremarked and unpunished. Veterans of the Japan Campaign would be ubiquitous - somewhere in the region of two million injured, not to mention the uninjured. Their experiences wouldn't be dissimilar to those of Vietnam veterans, but a generation earlier and an order of magnitude more of them.
There would be less questioning of why they went to war, but the US would be left with a huge scar. The baby boom would be dialled right back thanks to the loss of life, if nothing else. There would be more working women in 1950s America, purely out of necessity, and a lot of war widows raising children alone.
With Japan virtually annihilated as a country, occupation is inevitable. Guerilla warfare will probably continue indefinetely, but uncoordinated and ineffective. The Home Islands may be divided, or entirely an American affair - in either case, the occupying powers will have virtually a free hand and Japan will be a de facto colonial possession, with all that implies. Division between the USA and USSR seems both most probable and most unpleasant.
Korea is almost certainly occupied by the USSR, and will be handed over to a single (Soviet-approved) Korean government postwar. This probably won't be as bad as the OTL DPRK, but I wouldn't rule it out either. I'm not too clear on China, but I'd expect the continuation of the war to have some effect - the KMT will have retaken more of the south, but the CPC will have had more chance to build strength in the north. Soviet attitudes are probably key here.