Based on when I looked at this topic some years ago, the "enthusiasts" on both sides were over-optimistic relative to the actual state of affairs.
First of all, documentaries about the context of Japanese surrender and the Emperor’s speech make clear that his freedom to intervene for a capitulation was a near-run thing, so at the time it was quite possible that the invasion would have remained on track once atomic bombing failed to bring a prompt surrender.
1. No, longer. The extent of the defenses was seriously underestimated, and both sides had mistaken expectations about the other’s military capacities, how hard the other would persevere, and in the case of the Japanese the relative efficacy of willpower vs. hard power. See (2).
2. No. The invasion would have become slower and more deliberate as losses escalated and the density of defenses became more apparent, meaning efficient clearance would be more arduous and hazardous.
3. Don't know, maybe can’t know, how long it could be sustained even with accurate data on what was actually available for use then – certainly, the greater the effectiveness of US defenses in destroying incoming targets and generally causing attrition (including by bombardment), the more the Japanese would have to strengthen the assaults in order to seek to overwhelm the defenses to penetrate them. I am curious about the robustness of US fleet air defense readiness and whether the Japanese assault could be dense and complex enough to impair the US ability to ID friend/foe in the air and allow openings early in the battle by seeking to avoid friendly fire. Quite a mess for a time, I’d expect.
4. Operation Olympic was highly obvious but as an island gave the opportunity to cut the flow of reinforcements and supplies over the straits to a relative trickle, and therefore time to proceed more carefully and perfect tactical methods. With Japan’s leadership yet again gambling on breaking US will and using up a large portion of their resources there with no hope of evacuation, this would have to make the rest of Japan easier going, at least in the air. Wargamers would think about isolating and containing the forces on Kyushu through seapower and airpower in order to land at a more vulnerable spot on Honshu, but switching plans on the fly would not have happened in reality (the HOI4 battle planning system may create some of the same pressures). Certainly, the absence from Honshu of Japanese aviation and troops committed on Kyushu would make control of the air more manageable for the rest of Downfall. The Tokyo plain offered strategic importance and tactical advantages – relative flat terrain with buildings primarily of light and flammable construction that would make poor defensive rubble. From a strategic point of view, Hokkaido was relatively accessible to Soviet forces and a pre-emptive was feasible, but it would be a political diversion from the military task and it seems very unlikely Stalin would have overtly invaded Japan given Truman’s toughened attitude towards the Soviet regime and the greater importance to the Soviets of further gains in Europe.
5. No. With much less weapons skill, fieldcraft, discipline, and steadiness, I would expect civilian mass attacks to become uncoordinated on the approach and dissolve and even suffer substantial fratricide in the assault, making them more costly and less effective than charges by regulars. A relentless campaign of ambushes, demolitions and assaults by individuals and small groups were also part of the defense plans, and more likely, as in the Mideast today, to catch US troops in isolation, by surprise, or in a vulnerable disposition than would a conventional assault - a continual wearing campaign of psychological and physical attrition so long as the authorities could enforce their will on the people, where fanaticism would be blunted by exhaustion, hunger, and casualties.
6. Speculative. US losses would have been staggering by US standards, though not something to impress the Soviets. Japanese civilian casualties from combat effects, starvation and disease would have been impressive even to Stalin.
7. Once organized formations were smashed by ground and airpower and Japanese communications broke down completely, many would (as some soldiers did historically) fight on even in isolation until death or a recognized order to surrender, and force others to cooperate with their resistance. Such efforts might well have been supported covertly by the Soviets to stimulate continuing Japanese enmity against the US for the invasion and its consequences.
