Very little of the southern mythology upsets me. I've heard it all. But that doesn't change the fact that, as far as the United States was concerned, the southern states remained in the Union, albeit in rebellion.
Even records of the time consider their actions as re-admission into the union.
Well, then, you won't have a problem coming up with those actual acts of readmission, then.
If the States had not left then many things during the time would have been unconstitutional including Lincoln’s emancipation proclamation. He would have had no legal right to do that...
Have you actually read the Emancipation Proclamation? It doesn't free the slaves in a different country. It frees the slaves "within any State or designated part of a State, the people whereof shall then be in rebellion against the United States."