Have you read the Great Soviet Encyclopedia? I did in college back in the early 80's, at the height of the Soviet Gerontocracy. The entries on WWII outside of the Eastern Front in Europe made for good satire, but little else. In just a few lines, the entries on the war against Japan pretty much ignored everything except Russia's role. I can't imagine what even the most dedicated Communist at the time thought of the entry declaring that Russia had defeated Japan by itself in 24 days (they ignored the ceasefire on the 14th of August, and the fact that no fighting took place after the 20th). A war that in one form or another had been going on for years).
I've always felt it was the big shock of the Japanese Home Minister telling the Supreme War Council that Japan's economy would collapse (blockade, bombing) no later than October 1st, 1945 that really brought things home to the militarists. The idea that the railroads would stop running a month before Olympic started meant that essentially they would have their first wave of kamikazes against the invaders, and then that was it. Forces on the ground would have to fight where ever they were, with no chances of maneuver.
This was only days before the rapid falling dominoes of Hiroshima, the Soviet DoW, and Nagasaki. The FACT of the Soviet DoW meant any possible use of a powerful diplomatic go-between was now gone. The tactical results were irrelevant. Marshal Zhukov's tanks could have been caught in a typhoon and it wouldn't have changed anything. The horror of Hiroshima was bad enough, but the militarists had deluded themselves into thinking that the US had shot its bolt. That was not unreasonable for them. Any Japanese physicist could have told them the process for making an atomic bomb was extremely costly, and not possible to be mass produced.
But that was because they didn't know about the implosion device (U-239) developed for Nagasaki. That COULD be mass produced. It just took a lot longer to perfect.
Once all these factors fell together, the militarists really didn't have a leg to stand on. Ironically, the officer who put down the uprising was the very same Home Minister.![]()
As it happens, I did have the opportunity to read the (not so) Great Soviet Encyclopedia. I have actually used it in a class I taught on historiography, specifically to teach about how perception can be everything when writing history. It is good for a few chuckles though....
To your point, there is no question at all, that the Soviet DoW was a body blow, but for the most part it was a blow to the civilians who wanted to end things...it destroyed their hope that they had some sort of alternative to the Potsdam declaration. When the DoW was delivered, the super-hardliners (not the Big Six, but the real hardasses like Anami who didn't want to accept even the Emperor's decision if it meant surrender) saw this as the last alternative to a fight to the end (I was going to say gotterdamarung, but thought that might be a bit tasteless....), and actually took it as a positive sign.
I think you have to keep in mind how utterly 'over the cliff' past the point of no return most of the military was, and (especially) how terrified many of the senior military and civilian leaders were that some of the hyperfanatical junior officers might just take things into their own hands. Let me be specific...."Take things into their own hands" typically included the assassination (most of the time using swords, and all of the time being extremely unpleasant for the victims) of those that these officers felt were insufficiently dedicated to the cause. Remember that even a cursory examination of the last 25 years of Japanese history (1920-1945) would put a chill down the spine of most of their leadership contemplating surrender.
You are no doubt correct that most of these events (Hiroshima, Nagasaki, the Soviet DoW, etc.) were more than enought to convince any sane person that it was time to phone it in, but there were a significant number of Japanese military officers (and many of them well placed in the military in the Home Islands) that didn't see it that way. Anami led a revolt against the Emperor's express wishes on 8/15/1945, and whough he failed, it was hardly a foregone conclusion that he would. Anami wasn't insane (OK, he wasn't irrational-frothing-at-the-mouth-incapable-of-reasonable-thought, he might have been out of touch with reality by that time, but that is a different argument), and he didn't have trouble finding support for his efforts.