In a conflict between just China and Japan, Japan could have won. The western powers interfered numerous times blocking Japan from taking China. Western powers blocked Japan during the 1894-1895 China/Japan war resulting in China suing for peace. If not for the intervention, all of China would have fallen to Japan then. Same goes for WWII. Japan was fighting multiple western powers while attempting to take China. If not for the Brits and Americans (and others) battling on behalf of China, Japan may well have taken China in WWII. The Chinese did not put up an effective offense against Japan.
Maybe; it is all conjecture at this point. The biggest problem facing Japan was that they could never occupy China in the same manner in which they occupied the Philippines - they just didn’t have enough people. In the same way that the fall of Moscow would not have ended the war on the Eastern Front, there was no point in China that Japan could seize that would end the war. China started fighting Japan in 1931 (over Manchuria), and basically fought them through WWII, then with each other in their civil war, and at the end of that Red China fought us to a stalemate in Korea (22 years later, in 1953).
How was Japan going to beat them?