The rice crop of 1945 failed. Distribution of food was utterly disrupted. Atomic weapons were coming on line - 1 more in August, the infamous "third shot", yhree or four more in September, and three per month from October onwards.
http://blog.nuclearsecrecy.com/2012/04/25/weekly-document-the-third-shot-and-beyond-1945/
Downfall was likely to be put off if not outright canceled. A Soviet invasion of Hokkaido is entirely possible, but not until 1946. They'll consolidate and spend the winter getting ready for a Spring offensive.
So, taking larger famines from roughly the same time period, and going with upper fatality rates, a minimum of 15% of the population dies of food insecurity - ten million - over the winter of 1945/6. And that's without targeting agricultural production with chemical or biological attacks.
Going by the OP's assumption that the invasions are off - a good assumption, BTW - bombing fatalities is a little trickier. Back of the envelope type figuring...
The fatality rate of the actual bombing campaign varoes from source to source, but the 3-350,000 neighborhood is a good solid estimate. Most of those occurred between January and August of 1945. The firebombing of major cities in March, 1945 alone caused roughly 100,000 fatalities. The raids are going to become routine and effective, but the urban populations are going to drop due to evacuations and famine. Call it 50,000 per month from conventional fire bombing raids. 250,000 fatalities in 1945, 500,000 in the first six months of 1946, 750,000 total through June 1946.
For atomic bombings, I'm going with a lowered estimate from the first two for thr same reasons above for the conventional bombings. Call it 50,000 per bomb, three bombs per month. That gives us 600,000 in 1945, 900,000 in the first six months of 1946, 1,500,000 through June 1946.
So far, that's more than twelve million or 17% of the population dead by June 1946. At least another 7-10 million deaths before the end of 1946, if the country hasn't collapsed yet. And this is all before taking into account that there was starting to be actual serious resistance to the war and the military government in 1945.
Either the place falls apart entirely somewhere in the Spring of 1946 and there's no organized government to surrender or someone manages to displace the coup junta around the same time. In either case, tens of millions of civilian deaths and the country is utterly ruined, a wasteland with every city either burned to ashes or radioactive dust, industrial and transportation infrastructure absolutely gone, population starving, and no ability to to feed itself.
The knock on effects on the USAmerican psyche and the cold war are going to significant. The atomic genie is out of the bottle after the use of 30+ bombs on Japan. Stalin is going to be even more nuts. And he'll probably have all of Korea. China probably ends up divided, or worse an atomic battlefield...