I am sorry you are having so much difficulty with your soon-to-be mother-in-law. The fact that she lacks empathy for you and her son in these circumstances is a measure of her limitation. She feels affronted by both of you rather than able to understand that her behavior and attitude are offensive.
That having been said I think it is worth it for the three of you to have a talk. His mother got her back up because she felt you were trying to control whom she talks to. My encouragement is to explain that you were hurt by her reaction to your request, and her saying she would have a relationship with another ex of her son’s. I agree her behavior is disrespectful. Let her know that both of you have decided not to accept being treated this way.
Leave the ball in her court by letting her know that you simply won’t allow her to be hurtful to the two of you, but if she were interested in a relationship without the hurtfulness you would be open to it.
Then make a conscious decision to detach from her. In other words, don’t try to fix her, but instead have compassion. My sincerest hope is that she will rethink her position and be able to have a better relationship with both of you.