No, the most that you can say is that the southern states acted in good faith, believing that secession was legal. However, when the issue actually came before the USSC, they found otherwise. That isn't the same as saying that it was legal until the court said it wasn't.
Now, can't I in good conscience reject it, which I do. Why? Because returning to Lee's quote. The Union could be dissolved by 1) consent or 2) revolution.
But the southern states didn't have consent, and the general southern argument is that their actions weren't revolution/rebellion (since that would permit it to be legally suppressed). What we have instead are states seceding without consent and crying foul when their action is then treated as rebellion.
What of these words: When in the Course of human events it becomes necessary for one people to dissolve the political bands which have connected them with another and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
The list was long and so was the South's list against the North. Do you think George the III agreed? Do you think Louis XVI agreed? Do you think Czar Nicholas agreed? Do you think that Gandhi sought consent?
Lee wasn't saying first consent and if denied then revolution. He was saying the Union could dissolved by 1) consent or 2) by revolution.
Did they see it as revolution? I can't answer for everyone in 1861, but I will end with the words of Thomas Jackson: (Saying good bye to the Stonewall Brigade due to promotion to 2nd Corps)
Throughout the broad extent of the country through which you have marched by your respect for the rights and property of others you have always shown you are soldiers, not only to defend but able and willing both to defend and protect.
You've already won a brilliant reputation throughout the Army of the whole Confederacy. And I trust in the future by your deeds in the field and by the assistance of the same kind Providence who has favored our cause you will win more victories and add luster to the reputation you now enjoy. You already gained a proud position in the future history of this our second war of independence.
I shall look with anxiety to your future movements and I trust whenever I shall hear of the First Brigade on the field of battle it will be of still nobler deeds achieved and higher reputation won.
In the Army of the Shenandoah,you were the First Brigade. In the Army of the Potomac, you were the First Brigade. In the 2nd Corps of this Army, you are the First Brigade. You are the First Brigade in the affections of your general. And I hope by your future deeds and bearing you will be handed down to posterity as the First Brigade in this, our second war of independence.