Kanji is completely identical to Traditional chinese. They clearly didn't just "borrow" the language. They incorporated it into their own customs and culture. In addition, the Japanese received much of their culture and traditions from China in the Song/Tang dynasties as well. In the 1st centuries A.D., the people of Japan imported a wide range of ideas, techniques of production, institutional models, and material objects from the Chinese mainland. After adapting these imports to make them compatible with the quite sophisticated culture they had previously developed, the Japanese used what they had borrowed from China to build a civilization of their own. I really don't understand why you would reference the Neolithic pottery reference. Each had objects and ideas that the latter learned from and absorbed. Korean, Chinese, and Japanese history were all very much connected, and you honestly can't generalize a specific ethnicity from such a large part of the Asian Continent . Not all Chinese are "Han". They just identify themselves that way. Japanese are in no way ethnically uniform as well. The Okinawans were very related to the Chinese, and most of Japan most likely descended from Korean/Manchurian Ancestors. In any case, please do not generalize East Asian Ethnicities. I'm actually a mix of Korean, Chinese, and Japanese. Raised in Japan, moved to Korea for college. I know hangul, japanese, and mandarin, and I understand where each of these nationalities gets their identities from. The barrier separating us is not as big as some people say it is.
As others have mentioned before, this is not a subject worth arguing over. I think some of the debates have kind of derailed themselves. Whether or not we are closely related makes no large difference, in the overall perspective. We're all people, and we need to appreciate each other rather than fight over petty historical details. (And before someone gets offended, this applies to myself as well).