What is needed is a rock-solid Conservative SCOTUS willing to overturn earlier Liberal interpretations of the Constitution in such matters... shazaam... instant Constitutionality.
I would agree with you 100% there. But that doesn't change the fact that the secession was a rebellion according to the US Constitution.
It certainly was.
An approximate 1/3 of the country had decided that the Federal Government had finally gone too far, or was about to go too far, and sought independence for themselves.
The reasons behind their desire for independence were legion, even if the perpetuation of Slavery was very high on that list.
Part of the New Social Compact between North and South, to facilitate and sustain reconciliation, was to accord them the right to honor their heroes in that lost struggle.
Some generations later, we are now breaking that New Social Compact, by abrogating our own unspoken/unwritten obligations, and hindering their ancestor-worship.
We (the North) gave them (the South) the Consolation Prize of Respect for their bravery and sacrifice, even if we held true to our principles by NOT honoring their Cause.