Great analysis on Japanese foreign policy of the interwar era. I certainly did not know that much about Shidehara, despite my attempted analysis on the Empire of Japan.
It should be noted however that my analysis was so simplistic because its main objective was to answer the post above, which implied that if I only proved the existence of militarism in the 1920s my assertion becomes more agreeable. I did as such and my part was done.
Thus for answering yours I will attempt a deeper analysis of what I believe the foreign policy of Japan was being developed into.
The assertion given was that due to the complex nature of Japanese politics there was nothing inevitable about the outcome IOTL, which was militarisation.
First, I'll note that the question of whether or not Japan was a ultranationalist state was not dealt in the post, as the focus was on foreign policy and whether or not there was direct intervention on the path towards invading China(which, according to my assertions, is Japan's main objective). Hopefully it means my assertion on that part was agreeable, that given the proof of such Japan was in many ways more nationalist than other nations in the Western world - perhaps even since the Meiji Ishin.
This means I would now need to prove that Japan, despite some valiant individuals who fought against the rising tide, was lumbering in the general direction of militarism and seeking military ends to secure its assets and establish regional hegemony. I thus make the assertion that, as I have stated in posts of other threads, that Japanese militarism and ultranationalism is much more a bottom-up process, where if the government was seen as conciliatory in foreign policy, non-governmental elements rise up to stop it.
Examples that I am to give are hard to be seen as directly correlating to the development of aggressive foreign policy in Japan by the 1930s, but hopefully they suffice.
1. Genyosha, Black Dragon Society
Both of these are ultranationalist, semi-secret societies whose goals were to see a military-led expansion of Japan. Established in the Meiji Era, their growth and flourishing during the 20th century represent the increased ultranationalist, expansionist fervour of the Japanese people. Even without these sort of secret societies the Japanese often went out of their way to bring persons either unfaithful to the Emperor or unsupportive of aggressive foreign policy "to justice". One example is Mori Arinori, who was killed(1889) by an ultranationalist for having "not taken off his shoes" in a Shinto Shrine and pushed open the Sacred Veil "with a walking stick" - the irony being he essentially established the nationalist structure of Japanese education, based on neo-confucianism. Another is Hara Taakashi, last of the Japanese Prime Ministers that led the Seiyukai era of domination from 1900 to 1921. His assassination(1921), which was done by an angered railroad switchman, was most possibly due to his conciliatory policy towards Korea after the March First Movement.
This is not to even mention the activities of both secret societies, which include political blackmail, assassinations and terrorist attacks against those who did not support an aggressive foreign policy - and always got away scot-free due to their influence. Even during the Taisho Era the members were too powerful for the government to handle. This thus proves that Japanese militarist and ultranationalist ideology were coexistent and even flourishing throughout the first decades of the 20th century.
2. "Boy's Club" Magazine
This is the cover of the "Boy's Club", a youth magazine that led the printing industry for Japanese teen readership. The cover was from the 1921 November edition, the beginning of a 14-issue series that depicted the Yamato spirit. In the analysis of this magazine amongst others the author writes:
(emphasis added with underlines.)
The significance of this evidence is that primarily, it shows that even during the Taisho Era the ultranationalist fervour continued and flourished. As was with the secret societies, these magazines, the press and other civil organisations were in a relationship of mutual reinforcement with the general population of Japan, where nationalism and militarism developed and matured from a sense of weakness to a strong desire to reach greatness by whatever means necessary.
Both these evidences are certainly not to undermine the works of individuals such as Shidehara, whose effort and courage was, to say the least, heroic. However these movements are overshadowed by the fact that both a majority of the elites and the Japanese population were pro-militarist, who were willing to consider a military option if peaceful expansion was not viable - a firm determination only exacerbated by the Showa financial crisis and the embiargoes set by foreign powers.
I had a very long post half typed out to answer this, but then I realized that i had been working on it for days, and there was really no point. So, I will try to keep this short, at least by my standards.
The question we are debating, as I see it is whether or not Japan is destined to launch a full-scale invasion of China at some point. If that is not what you think we are debating, please let me know, but that is the question I have been trying to answer. In my view, Japan was NOT destined to do anything, and given the right PoDs, it could have gone any number of ways. I also don"t believe that Germany was destined to fall under Nazi rule and go on its spree of conquest, or, to go in the other direction, that the US was destined to retreat into "isolationism' during the 1920s and 1930s.
I don't think it makes sense to argue that Imperial Japan "always' had the goal of invading China. Certainly, it is hard to square that with the historical record, in which many different policies towards China were enacted over a short time period. One might be well able to argue that Imperial Japan had a long-standing policy of trying to expand its influence over China, when and where possible, but this is fairly normal for an imperialist power bordering a weaker nation, and not at all the same thing as an outright total war of conquest. And again, even this interpretation does not explain why Shidehara was able to champion the opposite policy for so much of the 1920s.
Similarly, I don't think the historical record supports the idea that Shidehara was somehow a lone man, standing valiantly against an ever-rising tide of opinion for the conquest of China. Certainly, the fact that he was in power (as FM) for so much of the period seems to suggest that he was not opposed by overwhelming forces. Particularly when, in 1928, his enemies did unite to successfully collapse the government of which he was a part, he was back in office in less than a year, and perusing the same foreign policy towards China, for a few years. Instead, I would suggest that Imperial Japan during this time period divided between two schools of thought on foreign policy matters, represented by the so-called Tanaka diplomacy and Shidehara Diplomacy. As I said in my last post, I think this dichotomy is overly simplistic, but at any rate there certainly was a debate between two sides, and not an acceptance of invasion, as you suggest. Even the Tanaka school, for all its acceptance of the use of the military to defend Japanese interests in China, was in the 1920s promoting the idea of using force to preserve Japan's existing interests (as seen in the response to the Northern Expedition closing in on Shandong), and not promoting the idea of a total war in China.
As to the question of whether or not Imperial Japan was already a hypernationalist nation prior to the rise of the Militarists... it is a hard thing to measure. Certainly during the 1920s, many people within the Military and within secret societies were promoting ultranationalism. Of course, many of these societies got their start in the late 1920s promoting respect for the military because of the perceived public loss of esteem that the military had suffered after the very unpopular Siberian Intervention. You and I have talked about Minobe before; his appointment to a government post,being allowed to teach at Tokyo Imperial University, and ennoblement in 1932 show that, as late as that date (that is, 1932) downplaying the importance of the Emperor as a part of the national government was not only acceptable, but not even a hindrance to one's career. this shows that, while aspects of Emperor-centric ideas already existed (and indeed they did 50 years earlier), they were not normalized or required prior to the Militarist takeover. You bring up the secret societies. I can agree that they had an influence, and tried to push the government to the far right. I do not see anything about them that makes the Militarist takeover of power inevitable, nor an invasion of China unavoidable.
In general, on the question of how nationalist the average man was during this time period... I would say that the Japanese average probably higher than that in the US or UK. In general, my personal belief is that Japan, German, and Italy all shared traits of higher-than-average nationalism during this time period, for similar reasons, including but not limited to the fact that all three were unified or reunified in the middle of the 19th century, and all three saw themselves as being disadvantaged by being relative latecomers to the colony-grabbing game. In 1931, there was fairly widespread public support for the rapid conquest of Manchuria. However, this has to be seen in the context of the time, in which many people thought the elected government was doing far too little to alleviate the crisis of the Great Depression, and any sort of seemingly decisive action was welcomed. I think it is easy enough to think of many different PoDs that would lead to less public support for this kind of aggressive foreign adventurism, as indeed the adventure in Siberia had earlier become deeply unpopular. At any rate, I certainly think there is a difference between (widespread but not universal( public support for what was seen as as a quick and easy military victory, and public support actually pushing the government towards war with China before it happened. Certainly, I don't see any evidence that the public was pushing from below for war, merely that many welcomed an easy victory after it had already come.
Lastly, I frankly do not see what "Boy's Club" magazine proves about anything. This short of literature, aimed at boys, was extremely common in any Great Power during the time period. Even your example mentions Lea, who wrote similar invasion literature books for the US market, with a great deal of "Yellow Peril" nonsense thrown in. Military adventure stories of these sorts, which painted the world as a dangerous place, glorified the national military, and wrote about the rightness of defending and even expanding the frontiers of the nation, were no different in Japan than they were in the US or UK. Particularly in the UK, stories written for young boys about young men, fresh out of public (that is, private) school, taking their place along the front lines of the Empire were so common as to be cliche. There is nothing special about Japan in this case. Actually, in Japan there was one famous young author (whose name unfortunately escapes me at the moment) who had toured Germany in the wake of her defeat in WWI, and wrote invasion literature in which Japan usually lost everything in a war with the US, and indeed showing war as suicidally folly in the new era. His books were well-received and widely read during the 1920s. If you would like to read a detailed analysis of these military-glorifying adventure stories for young men, in all three countries, I highly recommend
The Century of the Black Ships.
To sum up this post: I fundamentally disagree that there is anything inevitable about Imperial Japan launching a full-scale invasion of China. I similarly disagree that the same was a long-term goal of successive Japanese governments. I see no evidence for either of these positions. Instead, Japan tried many different foreign policies under different governments, and there was a lively debate within Japanese circles about which was best. In OTL, of course, the Militarists came to power, and they launched a program of total war in what became at attempt at outright conquest of China. However, there are Pods that would have prevented the Militarists' rise to power, and in those scenarios, no full-scale invasion of China would have been attempted.