Depending on when the Emperor of Japan (at the time, not a powerless figurehead, although they used their power rarely) and the Military leadership decided to surrender, I'd wager the civilian casualties would have been like nothing you could ever imagine. Once American troops set foot on the Japanese homeland with the war still raging, there essentially would not have been any civilians. Ever able bodied man, and likely woman, would have taken up arms against the invaders. It would have been wide scale slaughter on both sides, completely horrific.

Also consider what the average Japanese civilian or soldier knew of invasion. To them, it was routine to butcher, rape, and torture both civilian and military. Why? Because that is exactly what they did. Japan has much to admire about it, but it also was, and to an extent still is, an incredibly racist society, who believe that the lives of those not Japanese simply aren't worth as much. If you expect your enemy to treat you as you treat them, you're fighting with rocks if you have to to protect your country from the horror you would expect barbarian invaders such as the Americans to bring.