re: If a CPA makes a mistake that results in interest and penalties from the IRS....

You are in my experience responsible for your own return even if it’s prepared. Typically the engagement letter will explain how they will handle this. We had a couple instances where maybe we’d discount our fees for good faith.
Only thing I could think may change this is if the CPA could be shown to be intentionally negligent or fraudulent. But even then you probably are paying the IRS and trying to get it back elsewhere.