You make good points on the Lower North. My reasoning was based on the economic ties the region has to the Upper South and on the reluctance of the Upper South to secede IOTL. Much of that region was conditional unionist and that sentiment initially prevailed in the secession conventions. Indeed, for several of the Upper South states secession was presented as a fait acomplis. I'm thinking of Virginia, Tennessee, and Arkansas seizing federal facilities and signing alliances with the Confederacy before they had actually seceded. I don't think abolitionism is popular enough, or politically powerful enough to cause those states to break with the US, at least not initially. That said, if Chicago is able to push Illinois into secession, the southern portion of the state, Little Egypt, is likely to secede a la West Virginia. I think the same applies to New York City. IOTL NYC was quite anti-war due to the Irish population and commercial cotton interests. The city might actually try the whole "Tri-Insula" idea. There's no way in hell it would work and it would certainly be a spark for war ITTL.
I doubt New York City would even try staying in the Union in a Northern Secession scenario; the Irish there may not have been happy about the draft in OTL's Civil War, but most of them were no fans of slavery.
Little Egypt might counter-secede, but primarily out of not wanting to break the Union, rather than out of any pro-slavery sentiment, and I suspect the same sentiment would hold true in Maryland, Delaware, and Missouri as well(Kansas might be a different matter, though).
As for California and Oregon, I'm doubtful they'd go. Slavery lasted surprisingly long in the state IOTL and anti-black sentiment was very strong as well. This same point applies equally to the Lower North. Although, if the North is allowed to secede peacefully it would give a boost to the independence movement there. Maybe enough to actually try something. And if the West Coast does secede, either as an independent nation or alongside the North, the alt-US will discover just how hard it is to project power that far in the absence of a continental railroad.
Erm, what? Slavery was never present in Oregon and only a very small number of slaves ever existed in California, and there was little support for the Confederacy overall, even not much in those parts of the Golden State where slavery had been attempted IOTL. And apart from perhaps a few areas, anti-black sentiment in general(Joseph Lane's governorship in Oregon notwithstanding) was really not much worse than, say, central Indiana or Pennsylvania during this period; there was, in fact, a small but reasonably thriving black community in San Francisco during the 1850s.
The U.S. could try to
retake southern California, maybe, but I doubt that the Yankees would give it up without a fight. More than likely, all of California would remain in Northern hands, and certainly Oregon as well. Now, there may be the question of what happens further east of Nevada(if it still exists ITTL); it'd be interesting to contemplate the possibility that the Mormons in Utah might find themselves divided on the secession issue; perhaps a state of Utah in the north, and Deseret in the South?
Any and all of these would work. As would any POD that causes Lemmon v New York to be heard by the Supreme Court. However, my preferred POD, because it is the least drastic, is to simply accelerate the speed at which the case moves through the lower courts. The actual event dates from 1852 so it should be hard to speed up proceedings. Or failing that, simply have an identical case pop up earlier so it reaches the Court in time.
Maybe so.