This is actually a pretty interesting question, and to some extent I can't really give a definitive answer since this exact question is a topic of debate in the literature on the subject.
Probably the most definitive argument that the Second Sino-Japanese War did effectively ensure that the Communists would come to power was made in Chalmers Johnson's Peasant Nationalism and Communist Power (1962) which argued that the Japanese invasion aided the Communists in several key ways. First, the brunt of the Japanese assault fell on areas under Nationalist/Guomindang control (that is, much of China's eastern seaboard). Therefore, casualties were staggeringly high in Guomindang-controlled areas and in the Nationalist military. Second, the Japanese invasion played into the hands of the Communists' argument that they, rather than the Guomindang, were preeminently concerned with protecting the Chinese people from foreign threats. Jiang Jieshi (Chiang Kai-shek) famously stated that "the Japanese are a disease of the skin; the Communists are a disease of the heart." At the time of the war, this rhetoric would have seemed quite absurd to many Chinese living under constant fear of the Japanese. Johnson also argues that it is a mistake to conceptualize the Communist-Guomindang civil war as "ongoing" at the start of the Second Sino-Japanese War; in his view, the Communists had already definitively been beaten. However, by dismantling the Guomindang state, the Japanese created a massive power vacuum--especially in the countryside, which was in many cases already quite sympathetic to the Communist cause. Therefore, according to Johnson, the Japanese "restarted" the civil war between the Communist Party and the Guomindang government.
However, other historians have argued against Johnson's thesis. Most famously, Lucien Bianco's Origins of the Chinese Revolution (1967) suggested that peasant loyalty to the Communist Party was ultimately due to social, rather than national, considerations. That is, the Chinese peasantry genuinely felt that the CCP was responsive to its needs, as a class, whereas the Guomindang was ineffective at broadcasting its message to the masses. At its core, Bianco's argument is that the peasants were won to the Communist cause because of its efficacy at communicating its promises for genuine social change and its remarkable responsiveness to the actual needs and concerns of the rural poor rather than its perceived strength in the face of Japanese aggression. To put it succinctly, Johnson argues that the Guomindang "lost" the peasants whereas Bianco argues that the Communists "won" them. Their differences are not over facts, of course, but rather emphasis. For this reason, Bianco argues that the war with Japan certainly aided the Communist cause, but it did not solely bring about the ultimate victory of the Communists in 1949 (whereas Johnson does).
In terms of which explanation is more generally accepted by historians today, I am afraid I can't really comment. Both books have of course come under criticism in the decades since they were published. I think the trend among the historical community of China would be to weight Bianco's thesis slightly heavier, in that his research was more meticulous (he spent a lot of time researching the actual interactions between CCP officials and peasants; in that sense, his history was constructed from the "bottom up," if you will). But, plenty of historians would still swear by Johnson's general thesis.
Although my answer didn't exactly give a definite response to your question, I hope I have fleshed out the debate among the literature over this very topic.
If you're interested in any further reading, here are some sources that expound on the topic:
I'd argue it had a huge impact. Sino-German cooperation had an ongoing project that targeted by 1941 to have Nationalist China ready to compete with Japan on an industrial and military basis. The Japanese attack and Hitler's interest in retaining Japan as an ally against the Soviet Union caused the Germans to terminate this plan prematurely, including the arms purchases of quite a lot of heavy equipment (including cruisers, aircraft, and tanks). Were it not for the Japanese threat, the Xi'an incident would never have happened, and the Communists would likely have been crushed as an organized force. Alternatively, by the end of the Sino-Japanese War, the Communists alerady had a significant presence in North China, which was occupied by the Japanese and was relatively undamaged by the war (as well as being industrialized, for instance, in Manchuria). They also benefited from collecting vast amounts of captured Japanese arms and ammunition that was given to them by the Soviets. On the other hand, the KMT's prime industrial areas had been utterly devastated by the war, and Nationalist China itself was essentially dependent on foreign military aid (through the Hump and the Burma Road, for example) to keep its armies functioning.