Of course Japan could not unify China. They were being bogged down by civilian resistance. The civilians did the majority of the resistance compared to Chiang Kai Shek, who was bogged down by corruption, and Mao's forces which largely did nothing to repel the Japanese, conserving their forces until after the war. Guerilla warfare, waged like it is today in Iraq and Afghanistan is a very effective way of suppressing an organized armed force, because unlike a marked army, an irregular guerilla force can seamless fit into the population, and strike when least expected.
The words Japanese use are not the same as Chinese. Originally, Chinese script was directly borrowed from the unified Chinese script. However, during the periods following Japanese had to alter the spoken language. This is because spoken Japanese is not derived from Chinese (at least the kun part, which forms the majority of the grammatical structure) derived from the culture that existed in Japan. The thing is, during the Ice Age (40,000 BCE), when the Bering strait was Beringia, Japan was also fairly closely connected to Mainland Asia (a landmass that was a merged Korea and China) and SE Asia and Russia. The people that came from Asia became the Japanese. It is worth knowing that the oldest pottery in Asia is Japanese, from 10,500 BC, and is unique in the Jomon style, which used a corded pattern non-glazed as opposed to the earliest Chinese potter which was glazed a deep ebony, or painted.