Of course they were enslaved. Thanks for proving my point. Being forced to stay in a union at gunpoint that they entered willingly made all southerners slaves...well...except for the freed slaves who were allowed to come and go as they pleased.
Nobody...NOBODY...willingly joins a club that they do not believe they are allowed to leave, ever. The northern states believed that, until Lincoln came along.
Of course you can leave the club. Buy a plane ticket and leave. Bye.
Of course a state can leave the country, by amendment certainly, by federal law probably, or by successful case with the supreme court as original jurisdiction.
You can’t leave by assaulting the members, and stealing club property. You can assault the members, but there is a price for that. They might defend themselves. You can steal club property, but there is a price for that, where ever law and honor are respected.
For their illegal acts, Virginia et al. lost that part of sovereignty that they had not delegated to the US. That partial sovereignty was graciously restored at the end of reconstruction. Their illegal acts had no legal effect, per Texas v. White.