Not excusing what the Japanese did during WW2. But you are only considering what would happen during a war, not what would happen if WW2 never occurred at all. People do not experiment on war prisoners during peacetime, nor would a war economy necessitate draining resources occupied areas to feed the war machine. We need to little about little wars and peace not total warfare like we saw in OTL where unconditional surrender is the only option. Could Manchukuo develop its own identity under those circumstances. I think it could. Just avoid WW2. Hitler dies by tripping down a stairwell.
The problem is that China wants it back and shift in cultural/national identities is dicey and takes generations to accomplish in the best case scenario, meanwhile there's two major powers (China and the USSR) willing to smuggle weapons/aid into Manchuria, and maybe fight an actual war to take control of it.
Japan can keep Manchuria so long as both China and Russia are weak and distracted, but geopolitics and Japan's relative weakness on the Asian mainland means holding on to Manchuria is always a stretch
That was what I was implying all along. You have the Manchus, Chinese, Koreans, and Japanese all living together.
Manchuria was 95% ethnic Han by the early 1900s, the problem is that any Manchurian states are going to be overwhelmingly populated by Chinese even if you move some Japanese/Koreans in
That's not to say separate identities can't develop (it did in Taiwan), it's just that it's pretty hard for Japan to hold onto the territory long enough for it to develop
A russian/Soviet sponsored Manchuria would be another scenario altogether though