If she gets reinstated, she'll be reinforcing the rightness of the organization in her son's mind. And if she fades after being reinstated, she'll get no help from the elders with her ex trying to keep her son away from her. And your mom very well may be "encouraged" to shun her as well. So how about this?
How about relying on the law, the only thing that is real in this situation? She can go back to court and request custody or visitation rights to be upheld. In the meantime, you can make it clear to your mother that you and your future wife come as a package deal. If she doesn't like it, she can rely on that elder for more than gardening during her golden years. He can be there for her when she's sick or otherwise unable to take care of herself.