Good point. In the sixteenth and seventeenth centuries, the term "Natural Law" was used. So instead of saying "crimes against humanity," they might say "violating Natural Law" or "violating the laws of Nature," or something along those lines. It meant the same thing, but as you pointed out, laws of nature/crimes against humanity are subject to the morals of the period.Crimes against humanity is a legal concept from the 1940ties. By modern standards slavery is a crime against humanity.
But the concept did not exist in 1860 and slavery was completely legal.
(Just another example of how things like moral change. Another is the question of voting rights for women)
And if a state want out of the union they cango to congress and have them pass a bill saying you are out.
Since this is the procedure for adding new states, it make perfect sense to use the same for removing a state.
Unilateral secession the issue, not secession in it self.
I don't agree that Congress is a fail-safe for secession. Some states might not want another to secede, and Congress, by a simple majority in a lame duck session, might expel a state that would otherwise be in the Union.
It was the original 13 states that ratified the Constitution, and the the new states, via their conventions, who applied for statehood. Thus it is up to the states to decide if they want to secede. This is supported by the IX and X Amendments, and Madison's and Hamilton's agreement in the Federalist Papers that states retain all rights of sovereignty except those "delegated" or "alienated" to the federal government. Secession, of course, is not a right delegated to the federal government. That's also a reason for not seceding via Congress: it would be unconstitutional, because it is not a power delegated via the Constitution.