as per #4- it was as much the fault of that moron Douglas MacArthur that the Japs got off easy as much as anything else...from Unit 731 to everything else you mentioned, no one was feeling sorry for the Japs yet at the same time there was little pressure placed on them to really pay for their atrocities...were the world constantly reminded of Jap atrocities were constantly the way we are constantly reminded of the Holocaust, Japan would never be able to live it down...
Wisely, MacArthur vetoed Russian participation in the Japanese occupation and secured the basis for Japan eventually becoming a reliable ally of the US. MacArthur even provided for a constitution for Japan that was, by American standards, well to the Left, with key parts written by Norman Cousins no less.
MacArthur has much to answer for. I am not sure though that he and the US could have insisted on wide scale war crimes prosecutions without doing damage to compelling strategic interests.