IMO your granddaughter does not need to see a mother under the influence, and hiding the truth would not be an option to me. You don't like it when your daughter lies to you...do you?
She is doing what addicts do, no response to texts and so on. I'll bet that she will know your number when she wants something...they all do.
I know that all of this is painful to deal with, however, your grandaughter is the true victim of your daughters toxic behavior, do what is best for your grandaughter, your daughter has chosen her path, and she will live it on her own terms....not yours.