The fate of Japan
We've been round the block on this one before on here but it's always good to get a fresh perspective or several.
A number of thoughts to try and disentangle so here goes...
The Americans knew after the landing at Okinawa (or if they didn't know, certainly suspected) that any invasion of the Japanese Home Islands (JHI for convenience) would be a bloody encounter. In terms of airpower and ground firepower, the Japanese armed forces would be overwhelmed but without a concept of surrender and with the strong likelihood of civilians effectively throwing themselves with spears at the invaders, the death toll was likely to be enormous and very probably unsupportable.
The resulting famine alone would have decimated the survivors (in OTL, the winter of 1945-46 was a time of severe privation) let alone the loss of infrastructure (though many of the major cities had already been razed by firestorms with enormous loss of life).
From the Japanese standpoint, all of that was immaterial. They went even beyond Hitler and the Nazis in their sense of Gotterdaemmerung and self-destruction. The issue of surrender though was serious and the key point was the fate of the Emperor. On one thing all Japanese agreed, the Emperor had to survive. Tokyo knew that such a guarantee wouldn't come from Moscow but once Washington had informally tolf the Japanese that Hirohito would be maintained as Emperor, it offered the Japanese a way out with a modicum of honour.
The stunning Soviet successes in Manchuria in August 1945 had also transformed the strategic/military situation. I suspect no one either in Tokyo or Washington (or perhaps even Moscow) had believed that the Kwantung Army and Manchuria would be overrun in three weeks but that's what happened. With the advance into Sakhalin and the Kuril Islands, the Russians were far closer to the JHI than the Americans and the Japanese faced the real nightmare of a co-ordinated invasion of Hokkaido in the North and Kyushu in the south.
Now, the Americans certainly did not want the Soviet Union involved in any way in the final defeat of Japan though there was little they could do practically to stop the Russians taking land on the Asian landmass (including Korea). The one advantage the Americans had was the number and strength of amphibious landing forces but that advantage could and would be eroded quickly once Soviet factories started producing landing craft in numbers. In addition, Washington knew that if the Russians got a toe-hold in the JHI it would be nearly impossible to exclude them from the post-war settlement.
It was therefore the convergence of American and Japanese interests (keep Moscow out) that made the August 15th surrender possible. I think that even if a group of hawkish officers had staged a coup, the same thing would have occurred later.
The questions that are really interesting are: how soon could the Soviet Union have made an amphibious landing on Hokkaido (spring 1946 ?). Presumably, in the complete absence of an atomic bomb, the Americans, once Olympic had been concluded, would have massed forces for Coronet, the invasion of Honshu proper.
It may well be that in the absence of the A-Bomb, the Russians realise (as do the Anglo-Americans) that Moscow is playing a very strong hand and Soviet help in the invasion and occupation of the JHI would seriously reduce the potential American losses so a semblance of co-operation (liaison officers) is maintained despite the growing political divergence over events in Europe.
In March 1946, the combined invasion forces storm ashore, the Russians enjoy weaker opposition in the far north than the Americans on the Kanto Plain. Nonetheless, it will be the Americans who approach Tokyo first.
Endgame...the Emperor commits suicide as the Americans enter the city - a senior surviving officer is found who is prepared to announce a ceasefire bit fighting on Shikoku will go on for many weeks.
The de facto partition of Japan becomes de jure at the peace summit in Manila (hosted ostentatiously by McArthur). The Soviet Union gains the whole of the Korean peninsula as well as an occupation zone (Hokkaido0 while the rest of Japan becomes part of the American zone.
Before too long, the People's Republic of Japan is established based on Sapporo and entirely dependent on the USSR and the city of Hakodate becomes a cold war frontier zonbe as does the strait between Hokkaido and Honshu.
No Korean War but the Russians are more active militarily in east Asia.
In the late 80s, Mikhail Gorbachev signals the end of Soviet colonial involvement in the PRJ and Korea. In 2008, the PRJ is still ostensibly socialist but has been forced to turn to the rest of Japan for investment and aid. Tourism is flourishing as are calls for a re-unification of Japan.