The fact that the confederates originally wanted a federal right to own slaves is little different from them wanting states’ rights to own slaves. The fact that they changed from one argument to an incompatible one, and both were about the right to own slaves is just further proof that the arguments are simply disingenuous pretext. The civil war was about the right to own slaves.
The Confederate states had a federal law enshrining slavery. They had states’ rights to own slaves. At no point prior to secession did the federal government try to take a state’s rights to own slave or pass federal legislation abolishing slavery. They didn’t change arguments, slavery was always the priority, it was just opposing states’ rights while states were freeing slaves while the federal government was supposed to try to stop them.