This must be so awful for you and your family.
Sometimes we don’t get the parents we deserve, and sometimes as parents we don’t get the children we deserve. Life can be very unfair.
I think you have two distinct problems that need to be tackled separately.
How to deal with your DDs behaviour.
How to reconcile yourself to loving her and accepting that you can’t do anything for her and keep her safe.
For the second problem I’d suggest you and DH seek support from organisations that support parents of drug/alcoholic dependant children.
Id also suggest that you and Dh seek therapy to learn how to love her from a distance and how to cope with the worry that she will come to harm.
Then when you feel you are able to deal with this, you try and tackle the first problem ie how to deal with her behaviour.
I believe it is sometimes recommended by experts that removing parental support is better for the child in the long run. Perhaps if you are told this by a professional it will help you do what you know you must do - which is ask her to leave.
You could then give her notice - 6 weeks/two months whatever feels right to you.
At the same time, research the support she can expect to get if she is homeless/jobless/addict. Perhaps even contact these organisations now to get her on waiting lists for support etc.
Give her the list of organisations/charities you have found for her, (or even go with her to these people). Even if she refuses all help and does nothing, you will know what support is out there ie she won’t end up sleeping on the streets if she presents herself to get emergency accommodation. If you know how the welfare system runs you will feel more in control of what happens to her, and it may help to ease your mind.
Finally, if you feel up to it. You could implement strict rules and consequences while she is on her notice period. This is really a tactic to make her last few weeks more uncomfortable, so she is more likely to leave.
You and your OH are on a very, very hard road. You have reached a turning and your head is telling you she must leave because you have exhausted all avenues of help, and your own physical and emotional health can take no more.
But your heart won’t let you abandon her. Our children remain forever in our hearts as something valuable and precious that we must protect at all costs.
Your head needs to tell your heart what must be done.
Seek what professional help you can for yourself first. Then tell her to leave.