Japan would have lost, horribly.
And it would have been a blood bath for the invaders. I personally question how much of Japan or Japanese culture would have survived such an invasion.
If the US had landed in Kyushu as originally planned, they would have been fighting against about 1,000,000 well fortified troops; troops that were well dug in. The Japanese would have been heavily bombed and heavily shelled for months, and it still would have been hell for the invaders and death for the defenders. Just like the battle of Okinawa, the Japanese would have lost inflicting horrible casualties.
