I think I've read so damn much about WWII that my buddy, a history teacher, refers to me as the "Good Will Hunting" of the subject.
Currently working on Bellamy's "Absolute War", a detailed account of the Eastern front from the Russian perspective. Well done, with minimal errors that, once you learn enough, become glaringly obvious.
From all the stuff I have read, it seems clear to ME that the culture of total war had completely gripped the Empire of Japan, so much so that even AFTER the atomic bomb AND the threat of annihilation at the hands of the Russians, a strong faction of the military government came pretty darn close to even disobeying the will of the god-emperor.
So the assertion that the Japanese would have surrendered if left to "starve on the vine", as was the island hopping strategy employed by the US would likely not have worked. Would she have been impotent....probably. But I doubt she would have surrendered. She was pretty much impotent by the time the bombs fell anyway. Still lethal, but not on the scale of '39-'42.
Everything I have read indicates that the excuse used by the emperor and his cabinet was the "new, terrible weapon". And Japanapologists like to use it as a deflection for culpability in her awful warcrimes. But I think after studying it long enough, they understood that the Russians entering the war and steamrolling Manchuria was the real sign that things were OVER. The atomic bomb was the icing on the cake.
I'd say BOTH were significant factors, and it was easier for the leadership to use a "wonderweapon" as an excuse for unacceptable capitulation, rather than to come out and say they were scared sh*tless of the very real prospect of a "rapefest" as Fiz so accuratley put it, coming down from the Red army.
So put me down for both. And I'm pretty sure it indeed took BOTH to finally get that "new bushido" culture to get the hint.
Did you run your comment by Fiz, first? I'm sure he knows more about what you know than you do.