You can’t take Southern secession seriously because of slavery. Illinois is worth pondering because Lincoln supported the laws against blacks because he did not think that free blacks could ever mix with whites. A superior position was assigned to the white race. Lincoln meant every word. He voted against every suggested improvement for blacks. He saw universal emancipation as impossible.
As a moral solution he considered gradual emancipation along with apprenticeships, compensation to slaveholders, and deportation of Africans out of the United States. But Lincoln played the slavery card to ignite his own moribund political position.
As far as the federal government was concerned slavery would have continued as long as the states desired if the South had stayed in the Union. There was no serious opposition to slavery in any political party. The North did not invade the South over slavery. There was no single reason for states to secede. Secession was an American thing. The South was deeply American. The forty-year fight between the North and the South over tariffs was probably the prime cause of the war. None of this had anything to do with the moral issue of slavery.
The humanity of the South is missing from history. But, there were blacks who loved their country. American Indians who loved their country. Jews who loved their country. Secession in 1860 would have been a good thing. This needs to be understood. The forced union suffocated all. Centralism was imperialism.