If this isn't plausible, can you find a validating way to speak to your DD about this? "Everyone has a need for some time for themselves. I can see that you, as gd's mom, feel that she is ok to be left alone. I worry though, that you might get in trouble if something happens that GD can't handle and others get involved by accident. What do you think might be another possible.
solution for you in this situation?"
I like this and maybe give some possible scenarios like those given by thursday so she understands why it could be dangerous both for her daughter and for her.
Is your granddaughter's father in the picture? Could you work together? If you are unable to get your daughter to see the light, I would report her, as it is your granddaughter's safety we are talking about here. Your daughter is an adult but your granddaughter is a child. If there is no win (daughter) win (granddaughter) situation the way I see it, it's your job to focus on protecting your granddaughter. Do you live near by? If so could you give your phone number to your granddaughter so she can reach you if there is an emergency? While you work on this with your daughter.
I'm sorry that GD8 got in trouble with mom for talking to you about her absence my SO's uBPDxw does this with their daughters as well. Be sure to validate GD8, be sure to tell her she did the right thing. You need to counter the message that it was wrong that her mother gave her.
I hope your daughter is able to see the light.