My name is ***** ***** I'd be happy to answer your questions today. I'm sorry to hear that this happened.
I understand completely how you must feel, and I really wish I could tell you that there was a cause of action here, but this is one of the situations in life where the courts do not provide a remedy (other than divorce, which you're already doing).
The NY legislature abolished causes of action for alienation of affection
when they passed N.Y. Civil Rights Laws, Section 80-a. It's actually a felony to threaten to sue someone for alienation of affection (which is a bit extreme, but that's what the statute says). Section 83.
Generally, an employer is responsible for the negligence of an employee. However, there are two reasons that theory doesn't work in this situation. First, negligence refers to accidents, and having an affair with your husband was intentional. Second, because it's no longer possible to sue someone for alienation of affection, there can be no more vicarious liability on the part of the church.
I understand why you feel that she should be fired, and I honestly agree with you. However, separation
of church and state prevents a judge from telling churches how to run itself. Under the First Amendment, the courts cannot even entertain a lawsuit seeking to remove someone from working in a church. A judge can't tell a church that they're not following their own stated beliefs properly. Your recourse is to choose to attend services elsewhere, and let them feel the loss of your patronage (plus your children, if you wind up with custody).
I apologize that this was probably not the Answer you were hoping to receive. However, it would be unfair to you and unprofessional of me were I to provide you with anything less than truthful and honest information. I hope you understand.