Shore batteries might have been destroyed, but the Soviets were out up to 2,400 dead/missing and 1,000 wounded on the first day alone (they had less than 10,000 men overall). The entire island of Shumshu (let alone Paramushiro) was fortified ala Iwo Jima and between them there were over 20,000 fanatical defenders. For their part, Japanese casualties (according to their own records) numbered roughly 600, of whom 200 were dead or missing. In other words, the Soviets would have run out of manpower in short order.
Total Japanese strength on Shumshu was 8,500 men, with Paramushiro dealing with it's own Soviet landing by the 101st Rifle Division that was occurring at the same time (which there unfortunately doesn't seem to be much information on, it'd be interesting to compare and contrast the two, as well as the landings which were apparently resisted on Kunashir and Shikotan), and the Soviets suffered 1,567 casualties across the entire battle, not 2,400 at the end of the first day. Shumshu also distinctly lacked the sort of fortified caverns that typified Japanese fortifications on Iwo Jima, so to claim that it was fortified like Iwo Jima is facile. And with the Soviets having a beachhead solidly ashore of some 5-6 kilometers deep, having established contact with their supporting naval and air forces, with a steady flow of heavy arms being unloaded over a pier constructed by Soviet marine engineers, and with most of the available Japanese heavy weapons having already been destroyed, it's likely that the worst of the Soviet casualty rates for Shumshu were already in the past.
Bonus spoilers: I have several projects in-progress that I plan on posting here and on SpaceBattles, this being one of them - on 10 January 1945 the US Army Service Forces published a series of secret plans for large scale operations in Soviet Kamchatka and the northern Kuril Islands, specifically Shumshu and Paramushiro, which I have in their entirety. The conclusion? Just to occupy Shumshu and the northernmost fifth of Paramushiro would require four reinforced divisions with massive air support from Kamchatka and major fleet elements accompanying - 137,500 men, 19,500 vehicles, and hundreds of aircraft. This would entail the creation of not only a brand-new corps headquarters, but also an entirely new air force based out of Alaska and the far eastern USSR.
So in other words, a study by the US Army trying to posit what they themselves would have to use, completely ignoring the differences in methodology, supply demands, and distances between the US and Soviet armed forces. After all, it didn't take the Soviets 137,500 men to actually historically secure Shumshu. I bet according to this study, the Soviets should have been dead in the first few hours of the landing, instead of carving out a secure 4 kilometer deep beachhead in a day and securing a supply line good enough that they could bring in heavy guns after 24 hours.
By that same logic, August Storm by Glantz is likewise invalid as it is a contemporary of Sandusky's work. In the absence of any sort of compelling counter-evidence, I see no reason to doubt Sandusky's description of the situation. Indeed, as
@wiking has pointed out before by the time of the Japanese surrender the Red Army's offensive into Manchuria had largely come to a standstill due to logistical difficulties; the use of surrendered Japanese trains to occupy Port Arthur over a week after the surrender, for example.
Well, Glantz's original work is pretty invalid. However, Glantz has written updated editions since then that were published as recently as 2006. And no, the Soviet offensive had not come to a standstill at the time of the Japanese surrender: Soviet forces were still advancing and fighting across the board. Logistical difficulties were mounting and the advance was slowing, but it hadn't yet come to a standstill. With the Japanese abandoning northern and central Manchuria, there wouldn't be much change in the timing of the fall of those parts of the region. But undoubtedly the Soviets would have to pause to rebuild the railroads and stockpile supplies before they embarked on driving the Japanese from the southern redoubts they were falling back on, or perhaps just by-passing and isolating them.
Shumshu also didn't have 450 Japanese aircraft and 100,000 soldiers on it either. Given the VVS was only able to deploy less than 100 aircraft for Shumshu and about 200 for the movement into Korea, I'm content to say the Japanese definitely will have air superiority.
Hokkaido had around 50,000 men, not 100,000. What's more, you keep ignoring that the Japanese having 5-6 times the amount of men is rather offset by them having to cover
200 times the amount of ground. That the VVS only chose to deploy 100 aircraft for Shumshu and 200 for Korea is nonindicative of how much they choose to deploy for Hokkaido, particularly since by the time they do so operations on the mainland would largely be coming to a close and there wouldn't be much other demand for the aircraft to do.
In other words, after the Japanese ceased most resistance and over the process of days. As I said, the initial landings in Korea saw only saw a few hundred guys come off largely just the torpedo boats. It's rather easy to make a landing when it's not opposed, after all.
Word did not reach the local Japanese forces until August 16th, the day after the main landing of 5,000 men occurred, so no it was not after most Japanese resistance had ceased. The initial landing on August 14th consisted of 710 men, encountered heavy resistance, and was largely pinned within the port area by the time the main landing began the next day, which was done under fire. The bulk of the city was taken during the course of the day, but holdouts persisted on the outskirts.
Then the Japanese forces started to cease to resist.
Actually, as BobTheBarbarian has pointed out in the past resistance in the region largely and quickly did come to an end with the Imperial rescript and thus attempting to compare Post-Surrender landings to what would've occurred in a combat situation is a non-starter.
I can only find reference to one and funnily enough, when they needed additional firepower, they brought in minesweepers not more destroyers; that's very telling.
I've got one at the outset, plus another one brought in on August 16th along with an additional 2 minesweepers, 2 transports, and a patrol boat. No indication they preferred one or the other in the amphibious landing on Korea, although apparently there they were effective enough to knock out a Japanese armored train. For Shumshu, the bulk of fire support was actually provided by nearby coastal guns on the Kamchatka perimeter.
All of this ignores that the Soviets didn't have a naval gunfire doctrine like the Anglo-Americans either.
Sure, largely because their doctrine shunned the sort of assaults that required it.
[It's only a possibility if you accept that the Japanese do absolutely nothing for months which is, again, a non-starter.
So you claim, but we don't have any evidence that the Japanese perceived any threat to Hokkaido one way or the other. We also don't have any evidence to the inverse, mind you, but that still rather leaves things rather more up in the air then you are positing. For all we know, the Japanese might discount an amphibious threat against the islands and persist in their plans pulling forces out of Hokkaido to buff up Kyushu and Honshu.
As Bob has also pointed out to you in the past, they didn't misread Soviet intentions with regards to Manchuria and the Kwantung Army was in the process of preparing for exactly what was coming; many in the Japanese cabinet also realized how ridiculous the diplomatic play with Moscow was.
The leaders who pinned their hopes on convincing the Soviets to mediate (and the number of those were much more considerable then you are claiming) misread how willing the Soviets were in doing so and the leaders who recognized the Soviets were going to attack misread when and how the Soviets were going to strike (the western thrust through the took the Japanese leadership completely off-guard). So yes, the Japanese very much misread Soviet intentions, as well as Soviet capabilities.
As for Shumshu, you're ignoring that the defenders achieved a 1.5 to 1 ration in losses, meaning the 87th was destined to be annihilated.
As I've observed before: loss ratios mean fuck all. The battle may not have been concluded at the time the Japanese garrison surrendered, but all signs clearly point to it turning in the Soviets favor and the Soviets could bring reinforcements in at any time they choose, so the assertion the Soviet forces were destined to destruction is without basis.
Now consider that and realize there was three railways on Hokkaido, 100,000 Japanese troops and 450 aircraft.
Where did the extra 50,000 troops come from? Where are they on Hokkaido when the Soviets land and where are the Soviets landing? Why has the planned American bombing campaign to destroy Japan's rail bridge and tunnel network apparently suddenly decided to skip Hokkaido? Where is the fuel for those 450 aircraft coming from?