Nope. The states which seceded, did not do this by violating ANY of the provisions you outlined, the CSA was not formed until AFTER they seceded. You will find each state presented an article of secession, autonomous from any foreign entity. If the Southern States had gone and joined Mexico, or collectively declared independence from the US and loyalty to the British, that would have been against the Constitutional provisions you are citing. They each declared individual independence on behalf of the state, and the Constitution doesn't prohibit that. (or didn't at the time.) This changed with a SCOTUS ruling following the Civil War in 1869
http://en.wikipedia.org/wiki/Texas_v._White
So your correct, unbiased, truthful answer is... It was NOT illegal for the states to secede in 1860-61, and it was not "unconstitutional" to do so until 1869.
That is the most amazing demonstration of reading what one wants to hear I have ever seen.
Again: the CONSTITUTION states that all agreements entered into under the Articles of Confederation were still valid under the Constitution. It's right there in plain straight forward language in Article VI.
In the Articles of Confederation the States agreed to a PERPETUAL UNION.
Perpetual: 1 a : continuing forever : everlasting <perpetual motion> b (1) : valid for all time <a perpetual right> (2) : holding (as an office) for life or for an unlimited time
As such, the agreement to a perpetual union, maintained via Article VI of the Constitution, means it is UNCONSTITUTIONAL for a state to BREAK that agreement unilaterally. A court ruling on constitutionality does not mean it is unconstitutional from the point of the court ruling. That is a ridiculous notion. Does it take a SCOTUS decision to make a law prohibiting open criticism of government unconstitutional? No, making such a law IS unconstitutional with or without a SCOTUS decision. The courts merely VERIFY the unconstitutionality of a law. It is the language of the Constitution itself that makes it unconstitutional for the government to make certain laws or take certain actions.
Secession was unconstitutional from the point of each state ratifying the Constitution because the language that makes it unconstitutional was present from the beginning.
That said, the southern states certainly had a number of very legitimate grievances against the way the federal government was strangling their economy. It does not matter that many of the measures taken by the northern states were designed to make slavery an uneconomical practice. The federal laws that deliberately imbalanced interstate trade were plain, dead wrong. As such, via the principles laid out in our Declaration of Independence, a strong argument can be made that secession was the proper answer to a government which had shown a "a long train of abuses and usurpations, pursuing invariably the same Object..."
However, justification under the wrongs committed by the federal government coupled with the principles under which the Colonies had seceded from the British Empire, still does not make the action legal, nor constitutional.
Neither was our Revolution of 1776 legal if one wants to strictly talk legalities. Necessary, yes, in the opinions of our founding fathers, and with which most today would agree. But legal - no.
Therefore, the debate is not whether secession was unconstitutional, but rather whether violating the States' obligations to the Constitution (inherited via the Articles of Confederation) was justified with respect to the constitutional violations imposed on the states by the federal government.