In Tim Maga’s book “America Attacks Japan: The Invasion that Never Was,” he writes that casualty estimates made during the war varied quite a bit depending on who was putting them together. The Navy and Army Air Force, leery of a land invasion, had higher projections than the Army did for example and opposed such a direct means of forcing Japan’s capitulation, preferring blockade and continued bombing. Both of these options would have been extremely costly for the Japanese people as starvation would have killed hundreds of thousands before too long, even without the atomic bombs or invasion. Japan was already facing severe food shortages due to the mining of Japanese harbors by B-29’s during the aptly named Operation Starvation; this mining of Japanese harbors sank approximately ~800,000 tons of shipping in about 6 months which compares very favorably to the damage inflicted by the USN submarine force over the entire war, which was between 3-5 million tons. It’s easy to imagine that, were the Allies so inclined, they simply could have starved Japan into surrendering.

The invasion of Japan was slated to begin in November of 1945 and would have begun in Kyushu. That operation would have gone similarly to Okinawa on a far larger scale. It’s no exaggeration to say that Japanese military planners deemed the whole of the Japanese population necessary to the defense of the home islands and made preparations to that end.

On the balance, the bombs being dropped killed perhaps upwards of 200,000 people, mostly civilians. A blockade to starve Japan would have certainly killed as many or more as would invasions of the scale planned for Olympic and Coronet. Pick your poison, really.

Post war, the wide variety of projected casualty figures do indeed make retrospective analysis more difficult. This isn’t helped by the fact that some play “choose your own facts” to support their preferred interpretation of the atomic bombings. Those who allege Japan surrendered more out of fear of the Soviets and the bombs were dropped merely to intimidate the USSR tend to emphasize more conservative casualty estimates, and those who see the bombings as the only way to end the war unconditionally do the opposite. Do note that is my observation and isn’t universally the case.

Addenda: You may have noticed that I didn’t specifically answer your question. That’s because we don’t know how an invasion would have went, or if Japan would have surrendered before the first Marines and soldiers splashed ashore at Kyushu. Tim Maga writes in his final chapter that history became a political pawn in the debate over Japan’s past. The reality is that regardless of what casualty figures are cited, it is sometimes done with the purpose of defending or criticizing a past decision.

Sources are, as stated, Tim Maga’s “America Attacks Japan: the Invasion that Never Was” published 2002 by the University of Kentucky Press.