Mm. While I have great respect for Sinologists, I find that their information when they apply it to Japan seems to be... lacking. I say this because I was taught by a Russian Sinologist and also by Japanese nationals, and the differences were interesting, to say the least.
I don't necessarily believe that Shinto is 'syncretic by nature'. I believe that the Japanese are syncretic in their religious practices as a whole. If Shinto were inherently syncretic, why did the Nakatomi clan (whose purview was the Shinto system at the time of the Buddhist importation) fight tooth and nail to keep Buddhism out of Japan instead of incorporating it? They even assassinated entire wings of the fledgling Japanese state to do it. In part this was to ensure their own power base, but if Shinto were syncretic, then this battle would not have been necessary.
The Rome analog does not work, either. I do not argue that Japanese religious practice is very syncretic. I argue that it is impossible to know what purely Shinto (as opposed to amalgamated Shinto/Buddhist) beliefs existed at any point in Japanese history because by the time that we have written records on the matter, the religions are already side-by-side and some syncretism is taking place. This is not the case with Rome. We have written records for a far earlier period than in the case of the Japanese state, and I can only believe that the pagan Roman religion was syncretic.
However, one can also point to the syncretism in Rome's religion as a practical matter, as a byproduct of its expansion. Rome co-opted foreign deities to, in a sense, better assimilate its conquered peoples. An interesting point is that there is evidence in the Man'yoosyu that the Yamato state did the same _with the Shinto religion_ in its attempt to unify the different Japanese peoples. That is, Amaterasu was the 'clan god', if you will, of the Yamato clan, and so she is preeminent. Onnotengu, Susano-o, and the other Shinto
kami were given important but subordinate positions to co-opt them into the state.
Again, though, the records we have seem to be after the activities in question finished. They postdate the actual effort, and seem to be an attempt to use the new technology of the written word to make the cooption permanent.
As for your sources, you yourself admit that most of them are operating somewhat out of their bailiwick. While I do not doubt that their insights can be quite useful, especially in a comparative sense between China and Japan, I have found that there is a temptation to view Japan through China's lens, which most Japanese historians deny is a valid approach in the general case. I do not believe that Japan is quite the 'unique snowflake' that some nationalistic Japanese historians make of it, but it is clearly not a mere knock-off of China that such an appropriation of methods can fully explain.
I wonder if KO has any insights into this, as it really falls under his area of expertise.
