I definitely think that the US couldn't annex California now. The two cultures are just too divergent: between California being over 50% non-white and being generally a more liberal country than the US - no way. And militarily: California's allied with both Britain and the Empire of Japan, plus their own formidable military...the US wouldn't have a hope in hell.
I really have to stop you here. Just because it is more than fifty percent non-'white' doesn't mean they aren't white. Well, in a manner of speaking. Giving the marriages of tens of thousands (among the hundreds of thousands of) prospectors to Asian and native brides, I would say that we should say the Califorinos are mostly mixed. A bit more Metis than the cast a system for the Spanish Americas, and without the one-drop rule of the Americans. As for conservatism, they are still working on solidifying their own. Chinatown goes out of its way to keep some of their own buildings and designs around, even if it involves dismantling them and using them on fronts of their other stores as they move outward. Really amazes me how those old stereotypes of Chinese launderers and cooks went and how it made the two or three Sinai groups seen as such good spouses. Though of course there are still stereotypes, but nothing that seriously enlarged relationships with Nanfang, Chekiang, or related Asiatic countries. Was some trouble with Asian Spaniards from those islands, but once they sorted them you and found the alternative to Catholic Spaniards was Muslims and Hindoos...
Actually, quick question. If the US did annex California, do tpyou think they would have gotten the various islands the Californos did? So many were claimed by Marylanders and New Englanders for nitrates, whaling, and the occasional blackbirding, but I have to wonder if the Yankees would have enough sway or desire for it. After all, they did well out of keeping the railroads from reaching California as long as they did. Well, the time of New England and their ships had to pass for New Salem and their ports. Really though, it was never practice to get a railroad to California anyways. Even looking over the claimed borders of Texas, the US would have to start from Cotton Country and go through Mexican, New Mexican, Arixo, and Apache territory.