This is a great thread - much thought and scholarship. Even Kinana has risen above his usual rants. Congratulations. Let me add a little of what I know.
Not only were 19th century Chinese emigrants to the West, especially the US, mainly from the southern provinces, most were in fact from just 4 counties in Guangdong, collectively known as Seiyup (四邑 - siyi in hypy). They were pressed into working on the east west railroad that made the US possible. This is why for a long time, the US perception of the Chinese language was not Mandarin but Seiyup.
This brings me to the common misconception that the southern provinces had one language each, i.e. Guangdong had Cantonese, Fujian had Fujianhua, etc. In fact, the southern provinces were extremely diverse linguistically. The variants of Seiyup, for instance, are incomprehensible to most other Cantonese speakers. The same can be said for the more distinct forms of Shun Tuck (Shunde in hypy). Similar distinctions can be found in Fujian, Guangxi, Anhui, etc.
One could never hope to learn, let alone master all these languages. For that, I'm happy that there is a common Mandarin to facilitate communication among Chinese people in China. But what I cannot understand is why the PAPzis under LKY chose the option of linguisticide - murdering these other languages, or else denigrating them to the level of market dialects. This is cultural destruction.