Congratulations, NewGMA on your new grandbaby! Sorry DIL has been experiencing postpartum anxiety, but 2 months isn't very long for that to be going on. I'm sure she'll get past it in time.
I agree w/ those who say you shouldn't say anything to DS about her mental health. That's between them. And again, I'm certain she'll be better soon.
So glad you and she had such a warm, productive conversation. I understood your wanting to be "Mama and Papa" since DH's parents were, but I also get why DIL would object to that. I'm glad you've accepted "Grandma" instead. Please continue to keep in mind that they are the parents now, and you and DH are GPs. As a GP myself, I think it's a wonderful thing to be. Enjoy! And best of luck moving forward!