Launch services isn’t a contract to launch anything, it’s just NASA collecting information about every rocket. Rockets that blow up just get listed as Category I High Risk, allowed for Class D payloads that are cheap and replaceable.

They put a plug in the bottom that melts if the salt gets too hot and it drains out into a tank that stops the reaction with no moving parts or anyone controlling it. After it cools down they can remelt it and put it back in.