I am so sorry that you are going through this right now.
It seems that everything has compounded day on day for over a decade.
Many people would go under with just one of the complex, heartbreaking and devastating issues you are dealing with loss of a child, suicide, attempted suicides, chaos, serious MH issues and episodes across many family members, repeated abuse and neglect of a vulnerable child, significant neurodiversity issues (possibly undiagnosed) aging and depleted grandparents when issues are increasing. Potential exploitation (cuckholding / grooming / CSA) of a single mother and child.
Too much for anyone. Look after yourself first - get your head clear and restore some focus and energy before you make decisions / actions or take on more.
This might look like emotionally disengagement from your DD. Step right back.
Have you had EMDR for your PTSD? And how is your Bi-polar being managed.
Can you concentrate on you and your health and emotional well-being. Can you build positive happy times and connections with friends and family where DD and her issues are not discussed or dominating or preoccupying you?
Take care of your marriage.
Look for calm and peace.
Detach with love from your DD. Don’t inadvertently enable or fan the flames. Distance and grey rock.
When you have recovered your perspective decide as an extended family what you will / can do for your GC - and have a simple strategy that you all participate in a practical calm way to make is life safe and predictable.
I don’t see any point of your DH going round there. There is nothing to say that she will listen to. Stop trying to engage or rationalise with her obsessions and MH.
Dont get drawn in - observe from a calm objective distance and perspective and take actions when needed that as a family you all agree on (maybe with professional help) that focus on the GC.
Keep calm and practical in this crisis.