Prior to the Revolutionary War, all of the 13 colonies were slave territory. That was British law, and the colonies were under British law. After the war, about half the states banned slavery in pretty short order. I don’t know that any did so during the war.
I don’t know of anything in the Constitution that would be considered as ratifying slavery. The framers of the constitution did specifically bar the Federal Government from limiting the import of slaves for a time, to those states that decided to allow it. Each of the 13 states agreed to this, but it seems to be more accepting a necessary compromise than necessarily liking it.
I’m not clear on the meaning of the “validity of the principles of the Declaration” question. The declaration was never a law, and never had the force of law, but the principles it contained remained generally accepted.
In mentioning that only slave states tried to secede, I was referring to this: The first wave of secession was specifically to preserve the institution of slavery, by those who felt that the election of an abolitionist president increased the threat to slavery to a “too high” level. (Lincoln specifically stated that he did not intend to use the federal power to interfere with slavery in the extant states, as he considered that the Federal Government did not have that authority. But the tales that were told about him in some Southern newspapers made him an ogre that was going to attack any slave state, by means lawful or unlawful. How much the real Lincoln would have scared the southerners we will never know, but the imaginary Lincoln was considered so obviously unendurable that he buffaloed half a dozen states into trying to secede before he even showed up.)
The second wave of secession was specifically in reaction to Lincoln raising troops to re-establish federal law within the states of the first wave. The states in the second wave were willing to remain in the United States for now, but only if the United States granted that any state, which would include them, could leave whenever she wished. It was only slave states that considered it so important to open the exit, that they would fight any attempt to close it. Not all slave states were willing to fight to open the exit from the union, but no free state even considered it. How much the desire to protect slavery fed into their desire to open the exit, is not so easy to distinguish, at least for me. It certainly played some part.
In addition to Article I, Section 9, see Article I, Section 2.
And Article IV, Section 2.
Of the original 13 slave states, 13 of them voted to incorporate slavery into the United States Constitution. This included New York, New Jersey, New Hampshire, Massachusetts, Delaware, Pennsylvania, Maryland, Rhode Island and Connecticut.
Also, Virginia, North and South Carolina, and Georgia. Don't ever forget to cast four-thirteenths responsibility in that direction.
I'm not saying all the northern states liked slavery. If the inclusion of slavery in the U.S. Constitution had not been in their political and economic best self interest, northern states may very well have voted to exclude it.