Yeah, and the Founding Fathers never denied it. They knew that their actions were a rebellion, that British law did not support it, and that they would be forced to fight for their freedom.
A rebellion is by an unorganized regional group, whereas an existing government can lawfully withdraw from an already breached compact which it voluntarily joined without surrendering its sovereignty.
You keep insisting on that, as if one side deciding on their own that their actions were legal or that the compact has been breached. If the Union was a compact, then disolving the compact should have required the approval of a majority of the parties in the compact. Instead of doing that, the southern states merely walked out.
Again, you really need to read up on your history. Lincoln wasn't allowed on the ballot in all southern states and most border states.