Downfall would have directly wrecked the Kanto plain and Kyushu, as the targets of the invasion. This would mean a shift in the Japanese center of political and economic gravity back to the Kansai region, which while devastated by naval mines and bombing would be in far better shape for not having been fought in. Total Japanese fatalities were estimated at the time to be 5-10 million. Add in the famine that’s going to hit in 1946, with an estimated 11 million fatalities, and we can likely see a population drop from 72 million to 50 million.
In summary: devastating, and the country would be in for a long recovery, but not insurmountable.
As for Hokkaido, I’m leaning toward the Soviets being unable to take the island. For one, their plans were never to seize the whole island. For another, they didn’t have the shipping for it - according to Giangreco the Soviets estimated they’d need 12 divisions for the task, but not only could they only spare two, but they didn’t even have enough shipping to move both of them at once. Shumshu also shows that, regardless of the state of Japanese forces up north, they still had teeth.