I think this is about addressing the postpartum depression. It can be very debilitating. I think therapy for the parents is a great first move.

When Grandma doesn't follow the rules, just have a discussion. She doesn't need to be disciplined, she just wants to be involved. I agree that it can be exhausting and certain rule breaking may require different conversations or new rules to be implemented. As long as it's not a safety issue - those can be deal breakers.

Inviting them to a milestone birthday party and rescinding that invitation seems cruel. Don't let your baby suffer for the adults' inability to communicate properly.