I feel so bad for those of you with older children, it becomes so complicated!
I wonder if it might help if you were to use some MLC tricks on your kids. Afterall, if MLCers are like teenagers, then actual teenagers might respond to LBS strategies: "I am sorry you feel that way, I will be happy to discuss it with you when you care to speak to me with respect." validation without agreeing or tolerating poor behaviour.
Also, (and this is not an issue with my much younger children) but I think if they were older I would have a very hard time not "expecting" them to take sides. It is perhaps a reflection of your D's torn loyalties ( remember your issues with your H are not hers, although they affect her) that she feels she needs to sneak around rather than just be straightforward with her. Have you tried telling her that you accept that it is important that she retains a relationship with her father - let her off the hook for wanting to see him and the baby? I don't want to make assumptions, I am very aware that you maybe already DO all of those things.
In different circumstances, when I was a teenager my mother involved me in marital difficulties she was having with my Dad and I resented it. I KNEW my Dad had problems, I KNEW he was often an a**hole to my Mother, but I felt enormous guilt about taking my Mother's side and I was sometimes quite rude to her. And as a teenager, still trying to make sense of the world, I wanted ONE of my parents to be the grown-up and look after my sister and me. My Dad had issues, and my Mother was so caught up in her own problems with him that my sister and I had our own emotional development overlooked frequently and, it often felt, callously. I am NOT saying anyone here is doing this, it is just I can imagine with teenager and very young adults one can easily have unreasonable expectations of them in terms of loyalty and maturity.
As I say, I do not have teenagers yet, so I have no real insight other than being on the receiving end of my parents marital strife and knowing the huge conflicts it created for me, emotionally.