So, my real question is: did 瑞 used to be pronounced suì in 官话?
My roundabout way at getting to this is from the transliteration of Switzerland.
Let's look at Chengduese.《现代汉语方言音库 • 成都话音档》records 瑞 with two pronunciations. One being:
which could be roughly written in faux-"pinyin" as rui; which approximately (details aside) matches most MSM pronunciations.
which is essentially (for our purposes here) sui.
Now, the rule of thumb for 老派 Chengduese is that names and places are read sui and not rui. Which would make the pronunciation of 瑞士 something more akin to sui si - which sounds much closer to the actual pronunciation of Switzerland. What makes for an even better case for this pronunciation is a comparison to Japanese. The Japanese pronunciation of Switzerland is Suisu (スイス) and it's Ateji (当て字) is 瑞西.
How was Switzerland transliterated into Chinese? Was it through some topolect -or- did 瑞 originally carry the pronunciation sui?
edit: I supposed 瑞典 (Sweden) might also be an interesting case to look into - with similar sui/swe sounds.
edit #2: Looking at wiktionary Min nan/Hakka/Cantonese all seem to be much closer to sui than rui also.
edit #3: Wiktionary’s entry for 瑞士 does confirm some things:
From French Suisse, borrowed through a southern Chinese dialect (where 瑞 is pronounced with an s- initial). See also 瑞典 (Ruìdiǎn).
But it’s sorely lacking specifics.
edit #4: apparently 山东 topolect also reads 瑞 as sui.
edit #5: Grand Ricci also gives the pronunciation: shuì.