The timeline of Japanese surrender suggests that they were trying to buy some time to get a favorable deal and the Soviets made that a non-option by invading Sakhalin. The Japanese then immediately surrendered to the Americans, presumably seeing no path forward for a better deal. The timeline of meetings of Japanese leadership suggest the same. They did not rapidly respond to the nukes. Not surprising, as the nukes were no different in impact than the previous firebombings. They flipped out when the Soviets declared war and rapidly took South Sakhalin, however.