Similar to my recollection of U.S. History classes. I was taught that California was a Free State, and that caused a lot of arguing that led to the Civil War, because both sides wanted California to align with them. There was also talk of splitting California into two, with the northern half joining the Union while the southern half joined the confederates. Too bad that split didn't occur, as differences between the two halves are still there.
I think that your timeline is a little mixed up here, or you are conflating separate issues.
While the admission to the Union of California on Sept. 9, 1850 was one of the outcomes of the "Compromise of 1850," and its admission does follow the "Missouri Compromise" of 1820 (which bisected the nation at latitude 36°30' N and would have led to a "State of N. CA" and "State of S. CA"), California was actually not a "highly-contested property" in the Free State / Slave State issue, and there was never any real doubt as to the status it would have after admission.
There were other territories that were admitted alternately as Free States / Slave States in an ultimately futile effort to "maintain a balance." And, after the Civil War broke out, the Confederacy did suddenly lay claim to various western territories below the aforementioned parallel.
Regards,