Griffith, could you please expand on what exactly you do that constitutes "research" on the Second Sino-Japanese War, Chinese history, Japanese history, interwar Sino-Japanese relations, or WWII military history? You literally opened this thread by citing a manga as a historical source, and since then, you have not made any assertions backed by good evidence of any kind (pasting links to an entire book as "source" for an anecdote does not count).
This topic is of great breadth but there are some fairly questionable assertions that you made which jump out immediately.
1) Your claim that Chinese-Japanese relations were good in the 20s and 30s or that Japan had some kind of "hearts and minds" campaign before the outbreak of war is entirely unsupported by any kind of historical fact. After Japan's invasion of Manchuria in 1931, Japan and China were for all purposes in a state of undeclared war, consisting of brief skirmishes and uneasy truces, usually unfavorable to China. Hostility was mutual, particularly on the Chinese side, for obvious reasons. Even in the 20s, however, there was very little evidence of any sort of rapprochement or "hearts and minds" campaign, since Sino-Japanese relations were dominated by hostile events including but not limited to the Shandong problem, the Jinan Incident, the first Shanghai incident, and the assassination of general Zhang Zuolin. This does not suggest a policy of rapprochement or healthy relations at all.
2) You spend a lot of time dwelling on individual anecdotes like that one time a Japanese soldier married a Chinese local of how the Japanese colonized Mongolia with Han (colonizing Manchuria with Koreans would have been a better example btw). As Little Red Bean (as an aside, that is a very unusual username, I am not convinced it is not some kind of sexual innuendo) points out, this is a silly way to study history, because it is literally missing the forest for the trees. Deciding whether to colonize with Japanese or Han has basically zero relevance to historical trends. What affects who holds the levers of power does (a fact which is true of essentially all history btw), and the point of colonization is to transfer levels of power from indigenous control to colonizer control. Japanese colonization with Han migrants is not functionally different from Japanese colonization with Japanese migrants. Most of East Asia history between 1895-1945, btw, examined in this context, revolves around power, which is to say, Japanese struggles to upset American/British dominance of the economic levers of power, Chinese/Japanese battles with the West over sovereignty, literally all colonial conflict, anywhere, the list goes on.
3) Your usage of Chiang Kai-shek and other Chinese leaders desire to emulate and learn from Japan as evidence of a desire for a rapprochement with Japan reveals ignorance of Chinese political dynamics and the reasons for Japanophilia in Chinese leaders at the time. After 1916, albeit, really after 1895, the attitude towards Japan by Chinese leaders was one of admiration and fear, both because Japan, especially post-WWI, had raised her status to that of the premier military and industrial power in East Asia. Frankly, your usage of Chiang's frequent visits to Japan as evidence of pro-Japanese sentiment makes as much sense as claiming that monarchist German officers in the Weimar era were closer communists due to their cooperation with the USSR under Rapallo.
4) Your use of COIN and "hearts and minds" in relation to Japanese occupation policy is wholly anachronistic because it misunderstands the fundamental differences between implementation of COIN/hearts and minds respectively. Both COIN and hearts and minds are premised on the assumption that the ultimate goal is to win popular support from the local population to legitimize a military occupation; in a colonial war such as the SSJW, that is entirely redundant. This is also why counterfactuals such as "what if Imperial Japan less brutal" are poorly posed, they ignore historical context and essentially demand magical changes that hold all other factors equal. This is a fairly widespread phenomenon on this forum, to be clear. In the case of Japan, it not only ignores a decades long pattern of hostile behavior, but also betrays considerable ignorance of the manifold social factors in domestic Japan which strongly encouraged militarism (including but not limited to military influence on civilian government, victory disease, the rise of spiritual-ultranationalist cults within the military, a grotesque failure by Japanese general officers to grasp fundamental principles of diplomacy and economics, and the undermining of traditional authorities by fanaticism).
Frankly, the more I read, the more I question whether you have done any reading or research at all on this subject.