I fucking hate the cult the West has built up around its role in WWII, the war they idolize because it was about that one time they fought (more brazen and mask off) imperialists and genocidaires, rather than poor villagers with scavenged weapons somewhere in Africa or Southeast Asia. Not just because it sweeps some inconvenient details under the rug (ie, what sociopolitical and economic developments in America/Europe led to the Nazis in the first place) but because there hasn’t been a single war or geopolitical rivalry since that hasn’t tried to shoehorn in non-existent parallels, leading up to the big-brained take that a militant Islamist group comprised hardened orphans in a walled-off ghetto is somehow the Third fucking Reich in this scenario. It’s somehow even worse when they learn about the unsavory aspects of the (Western) Allies’ conduct because they can use that to justify war crimes so long as it gets the “bad guy” in the end.
Compounding this is a profound ignorance of the Israel-Palestine “conflict” as well since I’m sure anyone who isn’t a disingenuous fool would know about the Israeli settler-colonial system and put two and two together treating the occupied Germans and Japanese as an “undesirable” indigenous population to be displaced would have similar if not worse results. There were already Wehrwolf terrorist cells in occupied zones where the west treated “former” Nazis with kids’ gloves and poured in that sweet sweet Marshal Plan aid, just imagine how bad it would have been in that aforementioned hypothetical scenario.
There, I was holding in that rant for a while. 
No one is claiming that Japan would've "just surrendered" if the US "did nothing". The piece your missing is the USSR had finally started attacking and was sweeping the Japanese out of Manchuria. The invasion began 1945-08-09 same day as the second bomb.
Theres a leftist conspiracy theory that the Japanese high command actually surrendered because of the soviet invasion, not the atom bombs. But surrendered to the US not the USSR, since the US would probably be more favorable. The logic is the US had completely destroyed entire Japanese cities before but that never stopped them. Doing it with just one bomb was new, but thats it. Soviet invasion was basically game over. Holdings in China were doomed. And now they were looking at a joint US-USSR invasion of the home islands. Any general could see that's game over.
Alas its all a conspiracy theory. It hinges on if a bunch of japanse generals in a closed room decided to lie or not. I think one is on record as saying it was the soviet invasion but others hold to it being the atom bomb.
I've had people tell me multiple times on Hexbear that the Japanese government were about to surrender anyway even if the A-bombs weren't dropped.
You raise a fair point about the Soviet liberation of Manchuria, but like you said we really just can't know for certain what effect either or both of those events had on the Japanese decision making process. Both were war-losing events but without transcripts or recordings we'll never know.
What really speaks to me about the military's fanatical desire to fight to the end is the fact that even after losing both Manchuria AND getting bombed, the military still launched an almost-successful coup against their own God-Emperor to keep fighting. The surrender address had to be smuggled out in dirty laundry, and it's entirely possible that Japan would have gone on fighting had that recording been prevented from leaving the Imperial Palace.