WI Communists gain power in China, say 1928.
For the sake of the scenario they have a good relationship with the USSR.
1) Does Japan still invade China.
2) Does America take a less hostile stance to Japanese aggression against China.
3) Does Japan avoid any war against China.
4) Does the Pacific War not happen.
So imagining that the USSR and China are close allies.
5) Will Poland and Germany avoid hostilities for fear of the Soviet Chinese super bloc.
6) Will Britain and France be more conciliatory to Germany for the same reason.
1) They wouldn't. It would be a bloody endeavour, because China might wants some help from the USSR to solve the invasion problem. It would be easier for Japan to finance a coup, with opposents to the regime. Having a 1930-ish Communist China next to Japanese-controlled Korea would lead the Japanese to crackdown on any communist groups.
2) Because it doesn't happen, the answer is 'no'. America's oïl would go to Japan (which means no oïl sanctions). It might results in an early liberalisation of Japan (between 1940s-1950s) and the relations with the Western world (US, Europe, etc.) might be good. Old occupied territories might gain their independence: Korea could suffer from a communist revolution attempt, but it is certain the Japanese forces would help to counter it. So here, Korea's united.
3) The answer is... Maybe. It is 50/50.
4) No Pacific War, no Pearl Harbor, no 'hopping-from-islands-to-islands', no Midway battle, no Iwo Jima, no nuclear bombs dropped on Japan soil. Japan is, here, a US ally. The question is: would Japan intervene in Europe? Who knows. However, the European War might last longer. If the US are attacked by Germany (directly on its soil or indirectly), Japan will likely join the US in Europe.
5) At that time, China was very agricultural. Not industrialised. So, the balance of power doesn't change an inch.
6) No. WW2 would have still happened in this scenario, but it would have be an European War-only (unless, Germany attacks the US, then it is WW2, full power).