I agree that KO would answer much better!
I also agree with much of what you said. Actually, this was the main reason I answered as I did - we have no actual knowledge about any "pure" Shinto, untouched by outside influence. And the fact that Japanese are sincretic in their religious practice as a whole makes me doubt that any such thing ever existed. Certainly they periodically tried to fight Chinese influences - with varying degree of success. Even Vietnamese did so. And had a fact that their Bronze-age civilization surpassed proto-Chinese at some time on their side.
They are still heavily influenced by China culture. Japanese were influenced in less degree, obviously. But influenced they were - and their civilization started much later so its very birth was under the Chinese influence. Sometimes we can find out the exact degree of such an influence, but sometimes (expecially in early cases) we just can't. Of course, the mere fact of Japanese single Emperor dinasty and generally much higher degree of society stratification makes Chinese analogy not always applicable directly. Another thing is that sometimes one foreign influence was used against another - classical case is with Christianity, but Indian influences also show up in Japan iirc - and in Vietnam such conflicts were common (introduction of Buddhism in North Vietnam was stopped for a long time by the fact that North elites saw it as barbarian influence which shouldn't be allowed in civilized state).
As for heavy fighting against an introduction of Buddhism, I think that you already gave some explanation to that when you pointed out a relations of early Japanese religion with clan structure. It may be not so much fighting to ensure their power base as fighting against something introduced by enemy clan (not that one can always be clearly divided from another in such cases). This is a wild guess, of course - I don't remember that episode to any good degree and Japan is far from being the most interesting country for me...
And another fact which I remembered today and which is more on the game topic is that Japanese Buddhism also adopted some Shinto deities - depicting them in classical Buddhist way! So Indian-looking Japanese gods aren't so out of question!