Sounds like mom is breaking down, slowly. Just trust that she loves you and your family more than she dislikes the idea of you being poly. And it sounds like you're able to disconnect from the pain and anger and still love her and treat her as your mom. Very inspiring. Keep it up. You're an amazing woman and if anyone can make this work out, it's you.
I'd rather be hated for who I am than loved for who I am not.