So yeah, the Declaration is very relevant to the fact that secession is within the framework of the Constitution.
The three initial states, and the two subsequent ones are just further support for the same legal point.
Do you have anything in writing from say a state legislature or perhaps a US Congress to say secession isn't allowed? (And it has to be from around 1787-1789.)
Opinions of private folk are one thing, but official statements of governing bodies is quite another.
So does this mean we are finished with signing statements?