icanhandthemback I have only two children. I used to say that both read the baby advice books before they were born, one so that he could get everything right, the other to make sure that she knew exactly what to do to cause maximum disruption.
She was never violent, but we tolerated behaviour at home that we made absolutely clear was unacceptable with other people.
Fortunately she was born with tremendous insight into her own condition. By the time she was 20 she had decided she was too uncompromising to ever cope with having children or a partner, she has always needed a lot of time by herself.
She is in her 50s now. She has her own home, a good job, lots of friends, is a wonderful daughter and her niece and nephew love her to bits. But she has her home to retreat to, where she can be happily alone evenings and weekends, as well as working from home, when she can and we know, when we have family holidays to make sure she has a quiet space to retreat to and opportunities to go out without lots of other people surrounding her.
Is she ND? yes, I think so. DS and I are but are problems are easy to label. Like your DGD, we are not sure what shape hers takes
Has your DGC be seen by a child psychologist? We paid for a private referral. This was many decades ago, but we found it really helpful.
It sounds to me as if a full psychological assessment would be of better use to your DGD and her parents, than Social Workers, whose approach to their work is too often framed within their experience of mainly working with seriously dysfunctional families, so that they cannot see beyond that as an explanation for all family problems.