Correct. But the NMB has rules on when negotiations are at an impasse, they can't just stop mediating and park the negotiations. They will proffer arbitration which can be rejected by either party, and that's when the 30 day clock starts.

Anyhow, after the impasse, there is no such thing as "not be released to strike". By definition you are free to do whatever you want after the required cooldown period has finished. It requires an act of Congress to stop it, and they have never done it to an airline in the past, I doubt they will start now.