, very interesting, for a while (long while), I thought I was the only person who subconsciously then consciously and then systematically, analytically, and reasonably concluded this "hypothesis" as well. Your logic closely aligns with mine, and is also the sole basis of my "argument/hypothesis" as well; that is "The White Hmong - language and culture - if not the original, is probably the most closest to our ancient ancestors because they are the ones who first migrated out of China, keeping its authenticity and was/is therefore the least persecuted, oppressed, and sinicized by the Chinese." This unfortunately cannot be said of the Hmong-Chinese (Miao); all of those who still live in parts of China: Western, Central, and Eastern. And if you start comparing those Hmong-Chinese (Miao), as you have mentioned/noted, the most Eastern Hmong-Chinese (Miao) are the most sinicized and the further West you move, they are the least sinicized; evident in the Hmong-Chinese (Miao) of Yunnan or "Paj Tas Lag", because they speak practically about 90% Hmong intelligibly, exactly if not identical to SE Asian and Western Hmong. And for the most part, it is almost the White Hmong dialect, though there are noticeably emphasized Green Hmong dialect mixed within.
Recently, I finished reading a book by N. Tapp titled: The Hmong of China. Tapp makes a "point/statement" that the White Hmong - culturally and linguistically; those whom he originally encountered, field-researched, and had the opportunity to work with of SE Asia (Thailand and Laos) have all but immigrated to the West. And as far as Tapp is concerned or am aware, the White Hmong are "
extinct - that was his exact word" in SE Asia and especially in China - namely the "spoken language" he is accustomed to hearing and have heard/known throughout his research. (I was almost "floored" when I came upon and read this brief insight/statement and forethought by one of the premier Hmong researcher of our time. Though this is not the "smoking gun" to solidify the White Hmong; its language and culture as the authentic and original main stay of our ancient ancestors. It does raise "questions" that needs to be answered and/or researched.)
And you may or may not know this already, but the Green Hmong - several proponents - have already claimed that the Green Hmong; their language and culture/customs is in fact the original, authentic, and most representative of our ancient ancestors. Their argument for the most part is the "exact opposite" of your (our) argument. That is, the Green Hmong stayed behind, and not having to move/travel cross-country to make a living were able to keep, maintain, and hold on to their heritage - language and culture/customs. And because the Chinese left them alone for the most part, they were minimally influenced. At least, that is the logic or argument. Ironically, some White Hmong (elders) I have spoken with and raised/asked this "issue" with seem to agree with the Green Hmong for the most part. Some of them, the elders, claim/argue that this is the main reason why the White Hmong are the least populous compared to the Green Hmong. (I disagree. My logic/reason is simply that because, we - the White Hmong, were the most resistant to being sinicized and more rebellious; tried to escape (run away) and thus were the most persecuted, oppressed, and executed thus our dwindling meager number. The Chinese, in fact, separated/categorized the Hmong/Miao into two distinct groups - the cooked (obedient/submissive) and the raw (resistant/rebellious). It is a no-brainer who were the "raw" Hmong/Miao. On this note, the book: Haiv Hmoob Liv Xwm, written by a Hmong/Miao researcher/professor points to a war/battle where the Chinese annihilated over 50% of the Hmong/Miao population. (I cried reading this, and have watery eyes now just rewriting it.) But those that did manage to escape, rebuilt or tried to rebuild their lives throughout SE Asia - meaning all of us lively freely (ignorantly) today.
Lastly, you said your studies (sources/resources) showed the Hmong/Miao originated in Mongolia then immigrated to China. Care to cite those resources and/or scholarly researchers, documents, and publication? (I am always open/looking to expand my Hmong/Miao history data-bank and knowledge-base. Yeah, I have heard some folklore/tales alluding to Mongolia, but they are unfounded in my "humble/honest" opinion. I believe in the Siberia origination folklore/tale more because at least there are more scholarly works supporting that "hypothesis". Plus, you may or may not know this, but in the Hmong origination folklore/tale, it talks about 6-months of darkness in a blistering winter. Does Mongolia have such a climate - "winter solstice'. Nope. But Siberia definitely does. And interestingly, I saw a documentary a while back, and the aboriginals of Siberia bare a very close resemblance to the Hmong/Miao - mainly their lifestyle and culture, not language unfortunately.)