iac: The OP’s question has already been answered to her satisfaction, and that discussion has been exhausted.
NP:
“Halacha does not exist in a bubble. Even if a particular Posek is machmir for himself and his congregation. Even if he thinks HaRav Moshe’s Psak is an error. The issue remains categorically a dispute among the Poskim (about a din d’rebonon by the way). He can disagree but he cannot pretend Rav Moshe’s psak doesn’t exist.”
We can play your game and say – Halacha does not exist in a bubble. Even if a particular Posek is meikel for himself and his congregation. Even if he thinks Satmar Rov/Debrecener Rov/Rav Vozner’s Psaks are an error. He can disagree but he cannot pretend the Satmar Rov/Debrecener Rov/Rav Vozner’s psaks doesn’t exist.
“Now if he brings proofs against Rav Moshe’s Psak AND those proofs are deemed adequate by a majority of contemparary Poskim, AND it becomes the prevalent custom, then there might be something to talk about.”
Now if he brings proofs against Satmar Rov/Debrecener Rov/Rav Vozner’s Psaks AND those proofs are deemed adequate by a majority of contemparary Poskim, then there might be something to talk about.
Bottom line: You’re wrong. If someone is a member of a community or congregation that holds that Cholov Stam is Cholov Akum mamish, then eating CS is absolutely no different than eating CA for those people. There being other poskim who hold differently absolutely does not permit members of those kehilos to shop and use Psaks opposite of their own Psak.