SF, I empathise on the situation with your children. I have suffered on this one for a very long time. My kids are split over what they feel for their dad (again like your D of thier own accord) BUT it does make family life very difficult. Teens are not the best people to recognise and accept other's opinions at the best of times never mind during times of immense emotional stress.
I do still think H is being unfair to your S17 by using him to offload his problems. I know you say he isn't but if S17 is suddenly blaming you for all of this something has been said that neither of them are owning up to.
In my case I'm trying to work on getting my children to recognise and remember that they are siblings and that they should always be there for one another and that they should respect other's opinions even if they don't agree with them.
I know how hard this is. Take care
Me 43 XH 45 M 2.7.88 Divorce 7.10.09 Kids D20,S17 & D15