I'm with the "keep her out" camp. Here's why: Your mom has shunned you. She may have some ideas of "saving" her grandchild. When it is clear to her that you will not allow her to preach to your kid, she will be like all the other stories on here of JW grandparents who are in and out of kids' lives, hurting them in the process.
If she is a Patterson die-hard, she will be unable to restrain herself from making comments like "If daddy doesn't love Jehovah he will die at Armageddon." They hear those rotten experiences of tiny children saying "I don't want daddy to die!" then the inactive one comes back. You know how hard they believe that can happen. Even if you set a no-preaching rule, she's not going to do it in front of you.
Also, how will it be for you and your child when he is old enough to see that grandma won't talk to dad? Think of how distressing that would be for a child.
Exposing a child to a JW relative who is actively shunning a member of the family is damaging. You are the parent, and as such you need to protect your child from that kind of behavior.
Even though you don't blame your mother, because she is indoctrinated with WT rules, SHE does choose to follow them to the strictest extent. She has to bear some responsibility for that.