Misfire

Once there is a misfire the cat will get damaged. There is no such thing as a none cat damaging misfire. If you already replaced that cool and plug make sure u have good compression and that the fuel injectors are working

I would go to a different shop. Miss-fires will put raw fuel into the exhaust, this fuel burns at the catalytic converter and it can overheat and melt the substrate ruining the catalyst. It all depends how frequent the miss-fire is and how much fuel in not burning. A flashing check engine light will signal this to you.