from a legal perspective, it's fairly clear
There really is no "legal" perspective here. The question of the right to secede, at least in the Civil War, was a political, not legal, one. Law really requires the undifferentiated will of the populace. Any time you can accurately divide opinions geographically, you're outside the realm of law and into politics. Another good sign that your question is political is if armies are shooting at each other.
This, of course, has no practical application. Law and politics are almost identical. Law is just a political argument that's already been won.
The majority of the officers of the Confederacy were trained at West Point,
Actually, only 306 West Point graduates fought for the Confederacy. The CSA had more
Generals that that (401), let alone total officers. And the total men who fought for the Confederacy is probably somewhere around 1 million. That means that three-hundredths of one percent of those men had gone to West Point.
These men were not only traitorous criminals, they were liars and oath-breakers.
They had almost all sworn separate oaths to their home states. So, they honored one oath and dishonored another.
The mistake I think you're making is applying our modern conception of the union to Civil War times. Before the war, it was not at all decided that the we were Americans before we were citizens of our states. This is clear even in the way that the North fought the war. The Union army was composed almost entirely of state militias that operated as separate, coherent units. They fought under different flags and wore different uniforms (including New Yorkers who dressed like
this).
It really pisses me off when I hear someone describe Robert E. Lee as "honorable."
Can you imagine capturing a foreign soldier today, asking him to sign a pledge not to fight you anymore, letting him go and then having him actually keep his promise, retire from military service and refuse to fight? Because Robert E. Lee actually took that pledge and lived by it. He also attempted to lead some of his battles from the front line. On more than one occassion, his troops had to physically force him to the rear. After the war, he appealed to Sotherners not to begin guerilla actions, but to put the war behind them and rejoin the union.
I would honestly be hard-pressed to think of a person who was more honorable.