"Is it possible that the Shennongshi were non-Chinese, both ethnically and linguistically?"
Strictly speaking, no one was Chinese prior to the Qin dynasty which standardized the Chinese language.
"Would you personally describe them as non-Chinese?"
No, I want them to consider the Huaxia as less Chinese than themselves.
"Yue only became Chinese after the Han assimilated them."
Qin had already expanded into Yue:
but Qin and Han were ideologically opposed (anti-Confucianist vs pro-Confucianist), and residual discomfort with Han culture remained in former Yue territories even after the Han dynasty, as mentioned here:
https://trueleft.createaforum.com/mythical-world/yandi-vs-huangdi-myth-confirmed/msg12889/#msg12889It has been noted that Cantonese culture is traditionally marked by a "culture of distance from Confucius" (Jyutping: Jyun5 jyu4 man4 faa3; Traditional Chinese: 遠儒文化, literally "culture of distance from Confucius"),[49] which may be related to Lingnan historically being a fringe region of the Chinese Empire and influence from Tang dynasty (who placed greater emphasis on Taoism and Buddhism than Confucianism): While Confucian philosophy still has its deep influence on Cantonese culture, Confucius Temples are harder to find in Lingnan than Buddhist or Taoist ones.
so it could be argued that Yue is a more valid successor to Qin (and hence China) than Han ever was.
'it would make more sense to call the historical Yue Vietnamese ("Yue" and "Viet" are cognates after all,"
"Vietnam" literally means "South of Yue". Saying the Yue were Vietnamese would be saying they were south of themselves, which makes no sense. I agree, however, that Yue should feel closer to Vietnamese than with Han (let alone the further Turanized versions after the Yuan dynasty!).
"Shennong is labeled as a mythical Chinese leader"
I do not have a problem with this. It is the more Turanized whose Chineseness should be more suspect, considering that the categorical Qin conception of China was to infrastructurally incorporate the Yue (hence the Lingqu):
but exclude the Xiongnu (hence the Great Wall):
It was only the Han who disliked the Yue:
https://en.wikipedia.org/wiki/Baiyue#CultureThe Han also said their language was "animal shrieking"
Nothing has changed, by the way:
https://finance.yahoo.com/news/chinas-version-tiktok-reportedly-judges-090719443.htmlChina’s version of TikTok reportedly judges Cantonese ‘unrecognizable’