There are another couple of aspects to an invasion of Japan that were being seriously considered but generally receive little attention. Firstly the use of poison gases against both military and civilian targets. Secondly the use of biological weapons in any attempt to blockade Japan and starve it into submission.
Poison gas
Certainly the American public's view of using poison gas seemed to soften as 1945 went on and casualties began to mount in the Iwo Jima & Okinawa campaigns.
How public opinion influenced military policy on poison gas in World War II.
www.nationalww2museum.org
After the WW1 experience poison gas was a much feared weapon that the international community sought to limit if not eliminate via international Treaty, for example the Geneva Protocols of 1925 which the US signed up to but didn't get round to ratifying until 1975. One result of the fear of its use against the civilian population in Britain was the issue of gas masks to every man, woman & child in the country in 1938/39 and the development of gas masks for animals like dogs and horses.
To protect civilians from the effects of dreaded gas raids, respirators were issued to the entire population. Find out more about life in 1939 from Findmypast.
www.findmypast.co.uk
Gas masks for horses. The curator's choice
www.uu.nl
Had Britain been invaded in 1940 there was every intention of using poison gases against the invaders. Virtually all the warring nations in WW2 had gas weapons and moved them around in their rear areas as required. And there were accidents.
en.wikipedia.org
So while the article I linked covered the US position, the question remains as to how Japan might have used poison gases either as a first use against an invasion, or in retaliation. It had used them previously in China.
A Japanese scholar who focuses on tracing Japan's wartime chemical weapons legacy on Wednesday revis
www.bastillepost.com
en.wikipedia.org
Biological weapons
There are several stories about how Japan used these in China from 1937 onwards, and their development at Unit 731 (see above). In 1945 it even formulated a plan to attack California with biological weapons.
But the Allies too developed biological weapons in WW2. Amongst other things, Britain weaponised Anthrax and tested it. Churchill had to be reigned in from using it by the Chiefs of Staff.
en.wikipedia.org
The US also had a biological weapons programme. Had the option to starve Japan into submission been taken, there were plans to target her crops biologically to hurry up the process.
When you begin to look at the various options to defeat Japan at the end of the day and despite the casualties they inflicted both immediately and subsequently, the A-bombs, were probably the cheapest option in terms of the destruction of human life.