I'm assuming the ACC is based in North Carolina, which would mean there is diversity of parties, so the lawsuit would definitely end up in Federal Court regardless.
The purpose of a Grant of Rights is to avoid issues like sovereign immunity. Florida State already gave their TV rights to the ACC - they no longer have those rights. If FSU leaves the ACC and goes to another conference, the rights stay with the ACC and the ACC would be the ones with the rights to broadcast FSU home games. FSU would have to sue to try to get those rights back, which would make sovereign immunity mute, because they would be the ones bringing a lawsuit.
Now in practice, who knows what would happen if FSU threw caution to the wind, left the ACC and joined another conference, and tried to broadcast the games through that conference's TV deal. My guess is it would never happen because that conference would not want to get involved in a dispute like that. This is the reason that the SEC was all 2025 is great until Texas negotiated their way out of the GOR a year early.