Moomoola, this might be hard for you to hear as much as its as hard for your husband to hear, but I think it needs to be said now.
As things stand, you can only do so much. You can only wait to the point where you pick up the pieces. And I pretty much guarentee this will happen at some point purely because your daughter is 17 and its so very rare for what you are doing at 17 to be where you are at age 21 or 25 or 30...
You change so much in those years.
The single most important thing you need to do, is make sure she knows you are there. There for if/when things go tits up with her current set up.
Her life must be suffocating if she is being controlled to the extent we all think. And at some point you need to have faith that she WILL figure that out. SOMEONE will ask the key question or SOMETHING will happen when she will go 'why am I with this person'. And it WILL slowly unravel.
Precisely because we can all see this is a fundamentally unhealthy relationship, and unhealthy relationship are the ones that fall apart in the most dramatic of fashions.
And she will, most likely, have no where else to go.
This isn't the scenario you want, but its also a reality.
Your daughter could be doing all manner of other things you dislike and you wouldn't be able to stop her - drugs, hanging out with the wrong crowd, into crime, dreadful boyfriend etc etc. You'd not be able to talk her out of those things either. This is the age you kick back against your parents because you are programmed to - you are programmed to seek independence at this age.
In her case, she's saying this, but doing different. She's not looking for independence - shes looked for dependence elsewhere cos shes not ready for that independence yet but thinks she is. So the chances are she will get to the point of still seeking that independence just in few years time. And with kick back against where she is now.
Encouraging every single scrap of independence NOW will help with that process. The more she feels she is and can do things WITHOUT you, but equally without her other half and her mother the better. But also framing it as, if you ever need anything we are your safety net.
And thats all you need to do, and all you can do. As much as thats frustrating. Because you CANT control the situation. Your husband wants to control the situation but he can't. So hes trying to control you. You need to tell yourself this over and over again.
This is about what you CAN do, even if its only so very limited.
Genuinely, you need to be good to yourself and don't heap pressure on yourself to do the impossible. You can't change who she is and you can't stop her making bad decisions.
Her bad decisions are hers to make and hers to take ownership of at her age, whether you like it or not. Thats her right. As a parent, its your job to stand and watch her do it no matter how bad it is. Because you can't do anymore than that. She has to learn, and sometimes you can only learn the hard way in life. And age 17 is probably the very peak of that because you are so full of optimism and positivity. The cynicism of your ideals hasn't started to creep in yet. Disillusionment of politics and idealology comes to nearly all of us without fail though. And thats true even of cults.
You just have to feed the skills to be independent and fuel the fire of critical thinking. And that can be nothing whatsoever to do with trans. Or being critical of her partner. Frame it as being about HER as an individual. Independant. Who doesn't need anyone else (until she does).
There will be a big car crash, and thats what you have to hope for. And it will be painful. But isn't that everyone's first big love. And thats what this is - the all consuming nature of that which the school are too fucking stupid to see through that lens.
You need to gently suggest some of this to your husband, that yep she is allowed to fuck up her life. And yep its your job to be there when it happens. Its not your job to stop it from happening. Cos thats not how life works. Not when you are 17/18. Its a right of passage.
What were you doing age 17? What was he doing? What stupid shit did you do? Would you have listened to your parents without rebelling / questioning / getting upset?