Nowadays, there is a big body of law and procedure and tradition on how States are created and admitted (because it has been done 37 times). In the early days of the country, there was absolutely nothing beyond the words in the Constitution (Article IV, Section 3). These say that "New States may be admitted by the Congress into this Union;". Nothing at all is said about if a State, once admitted, can leave (or be expelled).And my basic argument is that:
Congress can pass a simple bill that say x area is now a state.
We also know that any changes to existing states require the consent of congress and the effected state(s).
And if congress (with consent from the state) can change a state and make it smaller, they should be able to remove it completely.
Some things have clearly been done which no one can find exact authorization for in the Constitution. The admission of Vermont and Texas might be examples. The Louisiana Purchase was regarded as unconstitutional, but too great a deal to pass up.
I happen to think, like you, that the power to admit a State implies that the Congress can allow a State to leave. Some would say I am inferring that and it is not implied. Strict constructionists might fight against allowing such a thing. Original constructionists might argue for either side according to what they think the Founding Fathers meant (or perhaps what they want the Founding Fathers to have meant). I think the Supreme Court would want no part of this tar-baby and would declare it properly a "political matter" for the Congress to decide; unless such an act was done and some State or States wanted to fight it in the courts, I think the Supreme Court would let the Congress do as it willed -- and then it would be an established precedent.
Accomplishing anything this way is all about political power and will. If "the South" could tie up the country's business in Congress in a time of great turmoil, they might force a deal to let some or all of the slave States leave in order to settle the stalemate. Doing that would lead to the other matters you mention, with negotiated conditions and terms. Separation would take a few years, most likely. "The South" would probably ask to continue the Missouri Compromise line to the Pacific as a border. The North (as in "the rest of the country") would be very concerned about transit rights and tariffs on the Mississippi River traffic.