Would China have been better off during the late 1930s and World War II if Mao had replaced Chiang as China’s leader in 1934?
This is very hard to know, but I would tend to doubt that replacing Chiang with Mao at that point would have made that much of a difference. I have two main reasons for saying this.
First, I do not believe that communism had the same kind of prestige in 1934 as it did in 1949. The Soviet Union was large, of course, but it was still fairly backward and had not participated in winning World War II. This meant that people would have been less convinced that communism was a good system. Mao had not yet had time to consolidate his reputation among the Chinese people. All of this means that Mao and the communists would have commanded much less loyalty and respect in 1934 and might not have done any better than Chiang in holding the country together.
Second, I do not believe that a unified China would have had much more luck against Japan than China did under Chiang. Japan was much stronger than China in its military and its industry. There was simply no way that Mao was going to be able to create a Chinese military that would be strong enough to fight off Japan in the time that he would have had available to him if he had taken power in 1934.
For these reasons, I do not think China would have been better off but I, of course, have no way to prove this.