So is there any chance that Japan can hold onto their pre-war borders in the north after WWII? Can the Allies be persuaded to let Japan keep the land, or is it inevitable that if the Soviet Union enters the war, those islands will be Soviet? Could fear of the Soviets post-war persuade the Allies to let the Japanese keep those islands?
The only thing I can think of is if Japan surrenders prior to the Soviet declaration of war, but would that be enough time to persuade the Allies for the final peace settlement? Would this ratchet up tension between Stalin and the West?
One thing I'm also interested in is the effects this might have on the Cold War/Japan as a whole. Would the Karafuto Prefecture-Sakhalin Oblast border be an intensely militarised zone maybe second to the Korean DMZ, where Russian defectors try to escape across? Could there be a "Sakhalin Wall" in the east? One thing that's obvious is post-war Japan would have one more prefecture than it does today, and the "four islands" of Japan might be thought of the "five islands".