As for "overwhelmingly", you make it sound like we're living in Mexico but have the indecency of calling it the US. As for the current state of population, LA is around 50% Latino at most. That isn't illegal immigrants. Those aren't people that identify as Mexicans in America. Those are Americans. Some even voted for Trump.
All territory was formerly part of someone else's territory. Borders move and people along with them. I feel no moral burden to accommodate previous inhabitants, nor would I expect any if the table was turned. That said, if states truly moved to nullify federal law based on a tenable legal position, I'd listen. However, they'd have to make such an argument, as opposed to a philosophical one (like yours, which I don't agree with).
As for your "understanding" of the origin of sanctuary cities, you've been reading fake news.
In our two party system, the lesser parties normally never get included in the legislative process. (those primaries effectively exclude them) In a parliamentary system any party that gets enough votes will get some seats.
If we adopted that sort of system you'd immediately see some greens and libertarians involved in the legislative process, and presumably you'd see an end to jokey pseudolibertarians in the Republican party, since everyone would just vote for the real thing. The "you're just throwing your vote away" disincentive would no longer exist. (based purely on how often that topic comes up in conversation during elections, I assume you'd see some big changes in the ideological makeup of the legislature)
I hate the Alt Right and the Ctrl Left pretty equally.
True, the Democratic Party is basically a coalition where the two main factions are the dominant center-right neoliberal faction (the "Third Way" that reached its peak under Bill Clinton and his since lost some ground in the party while still remaining dominant, if barely so) and the secondary center-left progressive faction.
As for sanctuary cities, when your state is sitting on territory that was formerly part of Mexico, its major cities still carrying Spanish names, its labour force overwhelmingly dominated by Latinos, it doesn't seem crazy at all to question the rationale of the US immigration system and borders and try to accomodate to the realities of the actual real world demographics.
Not to mention from my understanding sanctuary cities have their origin with the migrant fallout created by the central American wars that the United States was directly involved in.