Sometimes you need to lose the battle to win the war. I have two friends who have both trodden this path. Neither teen has had top surgery or takes hormones and both are 17/18 now, so the danger period for puberty blockers is well over. Both use their children's preferred pronouns and new names and have a good dialogue going with their kids, who still live at home and are off doing interesting education stuff. you admit you have not used their preferred name for 18 months, your husband not only refuses to do so, he's going round policing what friends are doing, such as this lovely mum who has reached out to help you (by the way, I always use the preferred pronouns and names of my teen's friends, given that pretty much every single one is non-binary or trans so they say).
This is not some type of collusion, it's accepting that unless you are prepared to take a step forward, a small one, then you are pushing your dd into a corner. This is likely to backfire.
Others on here will disagree. A similar situation which is nothing to do with trans is one of my children is fixated on having a boob job. Now I think this type of cosmetic surgery is a form of mutilation, not worth the risks, crazy if you want to breastfeed, and an affront to feminism and the acceptance of natural bodies. However, I don't say that ever to her. I just say 'it's your body, it will be up to you' and then, once talking, I have explained why it wouldn't be for me and why I'm worried about what it represents in a more gentle way. Now, she's nearly 18 and may well go ahead anyway, and it is her body and she has the right to make that choice, despite me thinking all the things I think.
I just think entrenching things like names, constantly worrying you are letting down GC thinking if you give an inch and your husband thinking others should be confronting your dd is the wrong way to go about it.
I don't think hearing from detransitioners that they wished someone had stepped is relevant here, because when people take bad decisions they regret, they look round for people to blame and it's easy to say you should have stopped me. Ditto bad marriages, bad boyfriend choices and so on.
I agree with everyone that your son needs attention, and that counselling is a good idea. I'd also be telling my husband that if it comes to his huffing and puffing driving my child away for good, I'd be leaving him but it sounds like that's not for you. No-one stands between me and my child, and given that your dd has reached out a few times to you (not how you'd like but reached out nonetheless, through the teacher, agreeing to meet then cancelling), him huffing and puffing and trying to control everyone is making it all worse. I'd seize on those chinks of light and cut him out temporarily, I really would.