I'm just going to copy the appropriate text from Wikipedia along with the obligatory references.
In my opinion, it was the accumulated devastation of firebombing that moved the Japanese towards surrendering, and the invasion of Manchuria that pushed them over the edge. The nuke attacks certainly helped, but even those were less devastating than some of the massed firebombing raids done by B-29s.
Sorry. I could not find a better word for what would happen and I should have been more clear. There would not be a full-scale rebellion against the Japanese government. But once the Americans invade, the Japanese will not offer a full-scale resistance to the last man, woman and child. Basically, things would fall apart, and my opinion is that most Japanese would simply capitulate to the American invaders (though of course the remainder would cause a hell of a lot of trouble). A few Japanese who were against their government would probably take the opportunity to try to assassinate government members.
Your 'substantial' evidence then, is a wikipedia article which in turn references ONE book by ONE rather fringey Japanese historian. The other references (and Battlefield barely rises to the term) make the argument (more defensible) that surrender was a result of MANY factors, not simply (or even primarily) the Soviet entry into the war. The notion that with the Home Islands isolated, Okinawa lost, and most of their cities burnt and blasted and two nuked, that the Japanese Big Six would regard the loss of some (not all, the Kwangtung Army lost Manchuria, but was still very much in business in other parts of China) their overseas posessions (which I stress again, were completely cut off from the Home Islands in any event) as the central reason for surrender is extraordinarily difficult to believe under any circumstances.
(later) I did take the opportunity to check out Hasegawa's work, and even he only makes the case that the Soviet entry into the war shattered the military's ability to resist, and thus empowered the civilians in the Big Six, not that it was the only relevant factor. More to the point, Hasegawa relies heavily on the post-war writings of the surviving civilians, who obviously have a very strong motivation to tell the story in a way that makes the atomic bombs look unnecessary (and thus paint Japan as a victim), which has been the post-war Japanese excuse to absolve themselves of their barbaric behavior during the war.
Regarding the idea that Okinawa, with a population regarded by the Japanese as roughly akin to 'hillbillies', despised and alienated, would be less likely to surrender than those on the Home Islands is simply unbelievable on its face. Your comment that the Japanese population subjected to weeks/months of heavy bombing would be at the breaking point is straight out of the terror bombing songbook so beloved by the USAAF and RAF during WWI, but once again, there is little reason to see it as anything more than unsupported assertion. The Japanese had no elevated crime problems, no serious unrest or resistance to military requisitions or draft, and there was no recorded violence against the Kempeitai, all things that would have been easily observed if their system was breaking down.
The death of the Emperor would have strengthened the hand of the military hardliners, not weakened it, as it was the civilians on the Big Six who were sustained by his 'behind the scenes' support, such as it was. Keep in mind that military hardliners (particularly junior and mid-level officers) were regularly assasinating insufficiently fanatical leaders (civilian and military) on a regular basis since the 1920s, so it is unlikely that without the Emperor's personal connection, some of the members of the Big Six wouldn't have met with untimely ends.
Finally, the death of the Emperor would have been a galvanizing force for the military, an event that they could rally around, and urge the people to rally around as well. This was precisely why the US went to such great lengths to avoid any action which would have a significant chance of killing the Emperor...