I got a headache reading your question!
Basically, your question is how your mom can provide your grandparents' marriage certificate.
Are you sponsoring your grandparents or your parents? If you are sponsoring your mom, you do not need to provide grandparents' marriage certificate. You only need to show your parents' marriage certificate unless CIC changed it in 2019 PGP. I do not see the relevance in providing your grandparents' marriage certificate, as you are only expected to provide evidence that you are indeed sponsoring your parents.
If CIC insists on getting your grandparents' marriage certificate, write a letter or sworn statement at a notary public explaining why you cannot provide grandparents' marriage certificate.