Two small points:
- US would never announce plebisite on Formosa. As far as US is concerned, there is already a legitimate Chinese government under Chiang, which they counted to one of the five victorious countries.
- Probably the same for Korea - Rhee Syngman was exiled in the US, recognised and designated him to be the head of government of an independent Korea. Roosevelt met with Rhee already back in 1941 and recognised him then as the legitimate provisional exile government. Rhee also attended and represented Korea in the San Fransisco conference.
So big NO.
The major point:
- Imperial Japan was by no means one single monolith. The (somewhat) more moderate IJN was irrelevant as a power as all its ships were already sunk and its prominent admirals were dead by the time of the Trinity test. IJA was determined to fight until the very end even after Hiroshima, and even Nagasaki, and the DoW by Soviet.
- In the Imperial Court discussing Japan's surrender, the Army (Gen Umezu), the Navy (Adm Toyoda) and the Minister of War (Anami) all opposed negotiating surrender, much less accepting the Potsdam Declaration (unconditional surrender). Home Defense was looking forward to prove themselves and preparing for suicide attacks.
- After the Emperor had decided to accept the Potsdam Declaration on the advice of PM Suzuki, Foreign Minister Togo and the IJN, a vast number of Army officers wanted to continue fighting even then. The Army officers staged a coup to siege the Imperial Household to stop a planned radio announcement by the Emperor, and tried to assassinate PM Suzuki. There were even minor incidents after the announcement.
Your question:
Under these circumstances, threat/offer of Hokkaido being seized by USSR is not exactly relevant. If anything, it speaks vaguely for the OTL, since the main reason for IJA to continue fighting was to negotiate peace on better terms after inflicting larger casualties on the Americans in kamikaze suicide attacks by civilians.
I also fail to see how this offer would have affected odds in the Chinese civil war. Sure, IJA had 700,000 men in Manchuria ready to fight the Soviets - but they were shipped to POW camps in Siberia, not affecting
Some alternative WIs:
- Japan declaring war on Soviet in 1941 (the planned Kantokuen attack) jointly with Hitler, with one million highly trained men armed with germ and chemical weapons.
- One of the reason why the Emperor suddenly turned on continuing the war was because he had been spared the actual state of play and never told about the losses in the war up to that point. Some argue Soviet DoW was more important for his decision than the two nukes. Assume he follows Anemi and Umezu and continue the war. The US might drop a third a-bomb on Tokyo at some stage.
ps. Sweden did not have an Ambo in D.C. 1944.