[4CJ]

Basically, the federal government has powers which the states don't have and that the states are forbidden from exercising.

Close. Read the 10th. The federal government has powers which have been delegated and enumerated, and those that the states are forbidden from exercising. Madison, in Federalist 45: "The powers delegated by the proposed Constitution to the Federal Government, are few and defined. Those which are to remain in the State Governments are numerous and indefinite."

Those powers are delegated by the constitution to the federal authority.

Wrong. The powers were delegated by the STATES (derived from the people of each state), and from the states to the federal government via the Constitution. The states existed before the union, they created the federal government. Or else you'd have to assert that the union created itself from the non-existing states.

No state can usurp them. But by seceding, the southern states were doing exactly that.

Wrong. No state in the union can usurp them. A seceded state is no longer part of the union. The states possessed those powers originally and delegated them to the federal government. They are not usurping the powers - they returned to the state when the political bonds between them and the federal governemnt were broken.

Again, I note that no state laws (including secession ordinances) can possibly take precedence over the constitution given that Article VI, clause 2 says that all state laws are subordinate to the constitution and that no amendment overrules the supremacy clause.

What, you think that if you repeat it a million times I'm finally going to agree with you? Pursuant. Don't forget the word. And exactly where does the supremacy clause state that no amendment can override it? Isn't an amendment part of the Constitution?

[Frumious Bandersnatch]

Let's break it down this way:

1). A state joins the Union.
2). By so joining, she gives up her sovereignty and accepts the constitution as the supreme law of the land.
3). Any laws that she makes from ratification onwards are subordinate to the constitution.  This includes any secessionist ordinances since they are also laws.
4). Any secessionist ordinance is a blatant attempt to put state laws above consitutional ones.  This is because the state is trying to usurp the powers reserved specifically to the federal government in Article I of the constitution.
5). The southern states admitted in most of their secessionist ordinances that they had no sovereignty under the constitution.
6). Using your arguments would mean that it would be legal for one person to secede (with all of his property) from the state and the Union and set up his own country.

Sorry, but your arguments don't wash.  If a local or state ordinance conflicts with the constitution, guess who wins?

Oh and BTW, it is exactly correct that the constitution takes precedence over state laws.  Period.  You keep telling me that this is true only if laws are made in pursuance to...  I have never used "laws made in pursuance to..." as one of my arguments (although I could).  I have made the constitution itself as the unconditionally supreme law of the land as my main argument (with some help from Madison and the CSA states themselves).  Neither the 9th nor the 10th nor any other amendment or article of the constitution overrule the delegated federal authority listed in Article I.  And that is exactly what a secession is trying to do.