What Constitutional argument do you submit that States are eternally locked in to the Union? The very antithesis of the Declaration of Independence.
What is the Constitutional process for secession? There is none given in the Constitution.
Both the arguments of the Confederate states and the arguments of Lincoln relied of two different sets of moral sentiments and assumptions (not law) they believed the Constitution relied on, but the clear legal language for neither was placed in the Constitution.
Yes, I think the Confederate states had the better philosophical argument, even though I reject their cause (to preserve slavery) but no I don’t think they were on any better Constitutional grounds than Lincoln; unfortunately due to the founders unwillingness to place legal language for secession in the Constitution.
It was always possibly to dissolve the union through mutual consent of federal and state governments. It still is. It could be done by an act of Congress or a Constitutional Amendment. It can't be done by a state acting on its own. There is no right in the Constitution for a state or locality to break with the Constitution and declare itself independent and free from federal laws.
The Declaration of Independence was written and signed when war had been going on between the colonies and Britain for over a year. It asserts a right to revolt and a right to independence, but says that "Governments long established should not be changed for light and transient causes." In other words, it doesn't say that you can throw over membership in a nation whenever you feel like it.