The Japanese language is simply a creole, as is English, as is Korean. Still, Korean is very clearly in the Uralic-Altaic group. Koreans can learn to communicate with Mongol people in Ulan Bator very quickly, as did the neighbor girl several years back when she founded the first Christian church to ever achieve government official recognition in Mongolia. She's fluent in Mongol and considered a very serious expert in Korean.
Japanese language differences are sufficient that I don't think anybody tries to categorize it as Uralic-Altaic anymore.
The Polynesian substrate is known, but it's reasons for being in Japan are not always obvious - unless, of course, you know lots of folks that look like the ancient Jomon pottery and who develop keloid scars!
Regarding the Shan, when Shan fell so did Mohenjo-Daro, Sumer, Syria, etc. Everything to the East of Egypt "fell" in some sort of natural calamity. Within a century there were Shan settlements on the Korean peninsula. Within another century there were signs that the Shan had made it to Japan. I don't think they planted an enduring civilization in Japan, but FUR SHUR they were there early enough to impress their genes on the original population. (It's probably easier to find the descendants of the Shan that it is to find the descendants of the 5 trade cities that were linked to the Silk Road culture.)