The D of I shows the steps necessary for valid secession:
1) should not be “for light or transient causes”
2) requires a certain “patient sufferance” while “evils are sufferable”
3) notifying and submitting the facts of abuse “to a candid world” (27 specific abuses are listed in the D of I) and finally
4) “when a long train of abuses and usurpations, pursuing invariably the same Object, evinces a design to reduce them under absolute Despotism, it is their right, it is their duty to throw off such Government.”
These are not a constitutional dictates, but, as the D of I says, what “Prudence, indeed, will dictate...”
The colonists suffered many decades of harm from George III and continually notified him of his wrongs and pleaded for redress.
In contrast, the South had not yet suffered any unconstitutional acts from the feds regarding slavery. The South should have first notified the feds of what acts were unconstitutional and why they were unconstitutional. Instead they ceded in anticipation of federal acts. There was really no “patient sufferance” and no attempt to notify the feds with reasonable constitutional arguments.
So the South’s cessation was invalid IMO and I believe the North had a constitutional right to fight them and get them back into the Union.
It shows no requirements other than a desire to be independent.
1) should not be “for light or transient causes”
Which is a suggestion, and further, what constitutes a "light or transient cause" is in the eye of the beholder. They thought their cause of independence was worthy. So did the founder four score and seven years earlier.
2) requires a certain “patient sufferance” while “evils are sufferable”
No it doesn't. You are projecting. Same with the rest of your list. You want the Union to be in the right.
Well they weren't. They don't have a moral leg to stand on.
So the South’s cessation was invalid IMO and I believe the North had a constitutional right to fight them and get them back into the Union.
The same right that George III had to force the Colonies back into their Union.
When the colonies seceded from England, they broke English Law. Law that had a lot longer duration than the US Constitution had up to that point.
Their argument was that "Natural Law" gave them the right. Well if natural law gave the colonies the right, then it gave the Southern states the same right as well.