In contrast, the Japanese knew they were almost certainly doomed if the US/UK had the conviction to press home their invasion of the home islands. They thought they could play for time by upping the ante through conscripting the entire population and fighting to the death in the ultimate scorched Earth policy. They thought they could raise the stakes enough to make a negotiated surrender an option, or stretch out the war long enough for some magical degree of luck to fall their way (e.g. Truman, a freshly minted president, making different choices than FDR, or a kamikaze run becoming super lucky and taking out an admiral or two and several key aircraft carriers, that sort of thing). The use of the atomic bombs made it clear that the US was not only willing to slaughter the Japanese as much as was necessary to get an unconditional surrender, but they could do so far more easily and rapidly than they had been doing prior (which was such that it would have brought about the annihilation by bombing of all major cities in Japan and the starvation of the entire population of Japan within the next year or two by mid 1945). Given that, they knew the game was up and let go of their last bit of denial that they could get lucky and eke out anything other than wholesale surrender.
The Soviets may have killed several tens of thousands of Japanese soldiers in Manchuria, but I doubt this weighed more heavily on the Japanese than the routine losses of soldiers and civilians they were taking from the US/UK forces (which amounted to tens of thousands per day, on average) on their doorstep and on their home islands.
""The Soviet entry into the war played a much greater role than the atomic bombs in inducing Japan to surrender because it dashed any hope that Japan could terminate the war through Moscow's mediation," said Tsuyoshi Hasegawa, whose recently published "Racing the Enemy" examines the conclusion of the Pacific war and is based on recently declassified Soviet archives as well as U.S. and Japanese documents..
"The emperor and the peace party (within the government) hastened to end the war expecting that the Americans would deal with Japan more generously than the Soviets," Hasegawa, a Russian-speaking American scholar, said in an interview.""
So even if the actual effect of the Russian attacks may have been small the fact that a country that seemed neutral to Japan suddenly joined the allies and attacked made a material difference in Japan's perception of the situation, in turn leading to a quicker end of the war than would have been the case otherwise.
1. The Japanese were hoping to reach a more favorable peace by USSR mediation.
2. Manchuria and Korea were providing essential resources for Japan's war economy.
3. Furthermore, after the USSR war declaration, Japan stood at risk of losing the island of Hokkaido for good, if they prolonged resistance.
You're not really addressing any of these points, except (tangentially) the last. Regarding that, the Soviet Pacific Fleet was certainly strong enough (and had plans) to mount an amphibious invasion of Japanese-held islands:
What I want to know is whether it was the determining factor of their choice to surrender when they did. Would they have surrendered as quickly or at all without a Soviet invasion? Would they have surrendered as quickly or at all without the use of nuclear weapons? My reading of history is that even without the Soviet invasion they would have surrendered quickly (at most within a few weeks of when they actually did). Without the use of nuclear weapons they probably wouldn't have surrendered as quickly, though ultimately I think they would have surrendered before a land invasion of the home islands but after much greater destruction of Japan's cities and much greater loss of life of Japan's citizens.
As to the point of the Soviet amphibious invasion capability: conquering a sparsely inhabited island with few fixed fortifications hardly is not in the same league as the battle of Okinawa and D-Day. The Soviets might have been capable of taking Hokkaido, but the US and UK were capable of taking Honshu, and they had the recent experience to prove it beyond a doubt. I'm sure the latter fact weighed much more heavily on the Japanese mind at the time.
It's not obvious to me that this was the case. In August 1945 most of the Japanese fleet was at the bottom of the sea, and they didn't have enough fuel for the rest. Ditto the air force -- there wasn't enough fuel, and pilots were poorly trained 9not enough fuel to train them).
The USSR in 1945 had thousands of effective fighter and bomber aircraft that could have won air superiority over the waterways needed to invade Japan. The distances an invasion would have to travel aren't enormous: 50 miles from Tsushima or 20 miles from Sakhalin. The Japanese would have found it difficult to send reinforcements against the Russian beachheads, because, again, they had little fuel, and Russian aircraft could have bombed the railways, putting them out of action.
Once the Russians had got a sizable body of men ashore, it would have been difficult or impossible for the Japanese to dislodge them: the Russian army was much better equipped and led than the Japanese army, and had just beaten the best army in the world. Japanese courage and self-sacrifice wouldn't have been able to make up that, any more than it was against the USMC.
The biggest difficulty I see for the Russians is whether they would have been able to cobble together enough ships to transport the necessary men and equipment.
Having said all that, Japan was deeply deeply fucked before Russia entered the war anyway.