I know, but how to find a way that Japan leaves that "half of China" alone?
Well, the Kantogun basically started that war on their own--the Japanese government back in Tokyo did not want a war with China. (They didn't want to conquer Manchuria, either, but that was already over). The 1930's in Japan is filled with incidents where junior military officers assassinate officials, occupy government buildings, etc. If you have the government really crack down on this--maybe after they kill Inukai--maybe they could replace disloyal/fanatical leaders and bring the military to heel. It won't be popular with a small but radical segment of the population, but if they spin it right--"they were disobeying the Emperor's orders", etc--I think they could get away with it.
If the civilian leaders remain in control. expect Japan to stay out of wars at least until WWII starts. When that happens, they will still likely be pressure to "not miss the bus", and grab European colonies while Europe is distracted. If this temptation is resisted, however, it should be a boom time for Japan. Japan experienced huge economic growth during WWI, no reason they can't do that again. Economic growth would itself greatly lessen the appeal of radicalism in Japan.
It is not impossible that Japan would even join the war on the side of the Allies. Maybe they are bribed into it. On the other hand, maybe they see the advantage of seizing Madagascar from Vichy France, "in order to prevent its use by German subs", of course. This would help repair the damage to relations between Japan and the West wrought by the Manchurian adventure.
Whether they join the Allies or just sit out the war, Japan will be in a good position come the end of the war. I see no reason the Cold War would not still happen, and Japanese Manchuria is a powerful check on Soviet designs in East Asia. It would be even more valuable if Mao somehow takes over in China even without the war. Manchuria has some good tank country--I would not even be surprised if the US ends up giving tank designs to Japan, so they can better resist a potential Soviet onslaught.
Right there, you have a Great Power Japan who is on good terms with the US and Western Europe. Might even be on better terms with Western Europe than the US, if the US pressures both of them to decolonize.