What are the chances that after a cat is cured of FIP, it will become infected again with a coronavirus that mutates again to FIPV? Should I treat a cat that secretes coronavirus?

Although theoretically possible, we have not yet seen the FIP-cured cats develop the disease later. Therefore, I would focus on curing your FIP positive cat. Do not treat a cat that only secretes enteric coronavirus. This cat has a very low risk of developing FIP and treatment with enteric coronavirus carriers will only promote the occurrence of drug-resistant strains.