ricecake said:CORRECTION ....
Only Chinese ABORIGINAL MINORITIES like Zhuang,Miao,Yao,Dai,plus others CAME FROM South East Asia,not Han Chinese who ARE MAINLY OF NE Asian stock same as Japanese and Koreans migrated through Tibet Kunlun mountains over 6000 years ago.
Han Chinese dialects belong to Sino-Tibetan language family.
Those SOUTHERN Chinese aboriginal minorities' languages are in the same Tai-Kadai group.
I don't know about language. But appearance and genetics tells us more in my humble opinion.
If you consider the Y-haplogroups, it seems plain truth that SE is an ancestral population of han chinese.
Korean and Japanese in genetics do not cluster with han chinese, that's very strange. The hypothesis of han chinese origin in south east asia is more plausible than your unsupported theory.