If you decide you want to leave the relationship, I recommend going to see a lawyer first, before you tell your H you want to end the marriage. Just find out your rights, what is likely to happen in a divorce, etc. The first session with the lawyer is usually free. They may recommend to stay in your home and let him be the one to move out, or to stay married for another few months to pass some kind of deadline that will be advantageous for you. Who knows. Just get the facts from the lawyer first, before doing anything.

Is there a way to block this woman from communicating with you? You need a strong boundary there. Can you block her phone number on your phone? Block her email so it goes immediately to spam and you never see it? If she sends you letters, don't open and just throw away or mark them "return to sender"? It's really inappropriate for he to be communicating with you in this way. She sounds like a weirdo. You can also tell H to tell her to desist.

Good luck.