Who cares what I say about this issue? Again, constitutional experts Thomas Jefferson and James Madison had clearly indicated that the states uniquely have the 10th Amendment-protected power to regulate immigration.
"Would you then agree that CA could allow anyone to immigrate, making them citizens of CA, who would then fall under the protections of Article IV Section 2?"
Your argument is basically the reason why the Founding States made the ”uniform Rule of Naturalization” Clause (1.8.4). If I understand correctly, before the Constitution was ratified, the individual colonies could make immigrants in their colonies citizens of the colony which also entitled them to the privileges of the Confederation.
But since it was known that it was easier to become a citizen in some colonies than it was in others, some immigrants used the ”easy” colonies as a back door to become a citizen of that colony and therefore enjoy the protections of the Confederation.
But the colonies with higher citizenship standards objected to this and when the Constitution was drafted the states agreed to include the ”uniform Rule of Naturalization” Clause to insure higher citizenship standards.
Regarding California non-citizen residents, there is nothing to stop California, except angry California taxpayers, from using its 10th Amendment-protected power to spoil immigrants with California state welfare all it wants to imo. But since Article IV, Section 2 of the federal Constitution applies only to citizens of the USA, not to non-citizens, the Cornfield v. Coryell case likewise addressing the rights of out-of-state USA citizens in New Jersey, other states are still free to apply their 10th Amendment-based immigration laws to non-USA citizen residents of California.
The clause in question says:
The Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several States."