No, substitute the words "unilateral secession" for slavery. It's quite a different thing. In Article I and Article IV the Constitution lays out a number of actions that a state may not take unilaterally, either alone or in concert with other states, becuase they may affect the interests of other states. The Constitution also requires congressional approval for any changes affecting the status of a state. Whether splitting up, combining with another, or the slightest change in its borders, a state must get congressional approval. Given this, the implication is clear that any action affecting the status of a state, or action which may affect the interests of another state, cannot be done unilaterally. That's where the south violated the Constitution. Secession isn't necessarily a violation of the Constitution, unilateral secession is.I find your argument very weak (you probably think the same of mine). Nowhere in the Constitution do I read that secession (or even your made-up term 'unilateral secession') is prohibited to the states.
You didn't answer my other question concerning whether it was legitimate or not for the Northern states to ignore Article IV, Section 2, Clause 3.
Nowhere in the Constitution do you find that secession is permitted, either. So you have to fall back to the tired old 10th Amendment arguement, and I maintain that the power to alter the status of a state, and the power to prevent any state or states from acting unilaterally any time the interests of other states are involved, are clearly powers reserved to the United States.
As for your second question, of course it was not permissible for the Northern states, or any other state, to ignore Article IV. On the other hand, it was also not permissible for the slave catchers to act with impunity. Slavery was immoral but it was also legal. Run away slaves, if caught, legally had to be handed back to the state that they wanted in. But that state also had an obligation to follow local laws in pursuing the run-aways. They should have had to file for extradition and go through the courts.