FSU, Clemson or whomever else wants to leave can stomp their feet and hold their breath, but there's not really much they can do to "make things messy."
The ACC has no reason to negotiate and there is no "win-win." There's win (4-5 schools that get B10/SEC invites), don't lose (4-6 to the B12), and lose (everyone else). Unless the schools that want to leave can find a way to make the GOR unenforceable, their choices are stick around or buy their way out, and buying their way out isn't economically viable for the time being.
If GORs were easy to break, OUT wouldn't done it. They paid $50m apiece for 1 year and that was with ESPN on their side. The ACC has 13 years left.