It was the US who dropped the bomb a week early. Russia agreed to fight Japan 100 days after Germany lost. They took the islands 100 days after as planned. Had we waited a week they probably wouldn’t have r at least not attacked as hard.
And the reason the USA dropped the bomb was to stop the USSR from invading Japan, and to stop the USSR at the 38th Parallel zone. Japanese cities had already been destroyed by conventional means (fire-bombing etc.) and the nukes were only a continuation of that. Japan's government realized they had more to fear from a Russian invasion than from the USA. By treaty with the Russians, the Kuril Islands were excluded from transfer to the USSR after WWII but the USSR took them by force anyway. By international law, the islands belong to Japan.