I’ve just read this whole thread over the last 24 hours and I think you’re doing incredibly @Moomoola. Calm and steady with dd even though you feel like screaming.
I think as others have said it’s really important to stay calm and rational. To make sure that whenever dd needs to (and at some point she will) there is a road out of this and back to her family. To do that though, she needs you to continue to be calm and non judgemental.
X has created a narrative where you are the evil transphobe parents and to keep her hold on dd she needs you to play that role. Partly because she has her own narrative (which she has probably been at least partly groomed into believing) that everyone hates trans people, they have to fight for everything because they are so despised. But also because it reinforces the “us and them” narrative she is feeding dd. No one understands dd like she does, only she loves dd really and only their love and relationship will get them through, it’s ‘us against the world’. All very theatrical for someone who loves the drama and needs to control and isolate dd. If you don’t play that part, she can’t use it to feed her story.
Sadly, I think that means your DH is wrong and his suggestions and behaviour will only push her away rather than pull her towards you.
I think you mentioned that dd is autistic/adhd and I’ve been thinking about how this might be contributing to the situation. Ignore me if I got that wrong but many autistic people and some people with ADHD esp the over focused subtype have an overwhelming need to control their environment. It’s their way of trying to navigate a world set up for NT people, that doesn’t make sense to them in the same way or is not experienced by them in the same way. I wonder if this could explain some of DD’s behaviour of arranging to speak/meet but then cancelling. She wants to do it but that means losing control and that makes her feel anxious so she avoids that feeling uncomfortable and regains control by cancelling. It makes me think that perhaps her talk about boundaries is her trying to explain this too? Does she have any PDA traits? It’s important to know then that this is not her being manipulative or playing games or trying to control you. It is her way of protecting herself from the stress and anxiety of interacting in the world.
Reading your posts I’ve also wondered about your DH and whether he is ND? Again, it would explain his ranting, lack of understanding/empathy for your approach, his demands that you do things his way and failure to see that firstly his approach might be wrong and secondly that he can’t expect you to essentially be his servant or his deputy, carrying out his instructions without question or ideas of your own. Has he always tried to parent dd in this way? Again, some autistic people (and children in particular) find what might be called ‘my way or the highway’ approaches difficult because it can conflict strongly with their own internal need to do things a certain way to help them function in an NT world. In some children it can lead to outbursts, meltdowns and outright defiance but in others it can present as withdrawal, shutting off relationships and shutting down. Perhaps if that is the case and your approach has been different that is why she engages with you but not with him.
Sorry that is a long post but just some thoughts on what you’ve said. I hope you find some time over the weekend to do something for you, even if just to sit with a coffee or have a bath.