We're really seeing the balance tipping away from the North with this. Unless Southern California just sees lots of antislavery settlers disrupt the plans...
The California Republic was established with representatives arriving from Shasta in the north of OTL's California state and Ensenada in the north of today's Baja California. The country's constitution acknowledged a "peculiar bond" with the US but the folks there opted against joining the union IIRC.
The constitutional convention of the California Republic had a firm majority in favor of independence. Borders were to be negotiated with the US, but Upper and Lower California were to be included. There was mention of a "University of South California Press" based out of San Pedro, which was annexed into LA in 1909 so that could mean a variety of things.
The only place mentioned as being US-dominated was the Gulf of California, not California proper. The Battle of Monterrey was in Nuevo Leone not California. I think it is more likely that we will see the US getting its pacific coast via Sonora. Coahuila, Nuevo Leone, Tamaulipas, Sonora, and Chihuahua were all desired by Polk OTL and it was Nicholas Trist going rogue that left them Mexican.