I know they managed to reinforce Kyushu as that seemed most logical place for "Downfall". I think they went from having 5 divisions on Kyushu in spring of 1945 to 20 by the surrender. Not sure where the reinforcements came from but mostly Honshu. Its only about 100 miles from Korea to Japan, so yes they would have suffered casualties but if we'd invaded they would have tried.You really think the Kwantung army could return t the home islands without being blown out of the water by the silent service?
And then there's the Air Force. They still had at least 5000 planes, many of which would have been used as kamikazee. Certainly some of that reserve was also tied down in Manchuria.