“What is stam halacha decided by other than precedent?”
In the case of Chodosh, an explicit pasuk. In the case of cholov yisroal, a gezeiras chazal. Was this even a serious question?
“And your incorrect about what chalav yisroel meant throughout history. There were many different ways of going about it. The Gemara wasn’t invented in Europe.”
I literally have no idea what you even mean by this. Are you actually trying to suggest that people outside of Europe had cholov akum and ignored the issur? Cholov yisroel is not an Ashkenazi minhag; it’s a halacha. If you’re talking about the heter of the Pri Chadash, then fine, but I’m just going to call that a kulah as well because that’s what it is.