The fact that your mother had lunch with you knowing your situation is proof that she WANTS to have a relationship with you isn't it? She WANTS to show love to you because that is the way we are made.
Just tell your mother that you love her unconditionally. And, just as she has the right to decide how she lives her life so do you but that does NOT affect your love for her. Let her know you will always love her regardless of her religion. Then, ask her if she REALLY thinks Jesus believes we should disown our family. His brothers were not believers and yet there is NO proof that Jesus ever shunned them. And, they had Jesus himself in their family!
I agree that in your situation I would not pursue legal recourse. The religion is going to do what they do and it only highlights your opposition. That is probably not the best thing for your mom to see. I think your mom is really hurting too. She probably only reported you because she thought she had to. Your continuing to tell her you love and need her will do the most to break down this unnatural barrier her religion is imposing on her.
I wish you the best. You never know what can happen.