Personally, I would not die on the hill of names and pronouns whatsoever. The most important thing in all these situations, and I've seen lots over the years (many not involving trans, but 16/17 year olds running off with unsuitable drug dealer boyfriends and so on), is to keep the lines of communication going. I have a friend who has trodden this path; she uses her child's preferred name and her child knows she's gender critical, but she's able to communicate and speak to them by respecting their name, it's quite a simple shift and not even all about trans- in Breaking Bad Walt Junior decides to be called Flynn! Two years in, it should be quite easy to remember, people have been changing names in teen years for ages.
I think teens often are seeking something, one of my good friends went all religious at this point and asked us to attend happy clappy churches with her, we just kept our own beliefs, went once and it burned out after a couple of years.
Just ask yourself not what do I believe but how can I keep good lines of communication with my child and an open house so they are always welcome. They do do things we don't like, or are afraid of or we think are silly or dangerous, I still think that keeping the thread of connection is important.
No police force is going to come in and take a 17 year old away from where she chooses to live, if she says she's happy with the partner and the mum. They just aren't and if they did, your child will run back. What would be achieved?
I have fallen out with my dd over trans, I don't discuss it with one of them, the other is GC anyway, but we operate a 'this isn't worth breaking up our family or our friendships over'- half my dd's friends are trans or non-binary (in their heads), we discuss it but our goal in life isn't to convert or impose our beliefs this is silly on them, but to have friends, to live our lives.
Being a 'constant' doesn't mean holding a line which might risk losing your child, there's almost no things I'd be prepared to do that over, and this gender issue, which will fade away in 10 year time isn't worth it IMO>