Ah man this really sounds a lot like how things started for DD, and the same kinds of things went through my head too in terms of whether she was doing it to spite me and stuff.
I'd say I'm a stage or two ahead of you and I can say that we had a huge falling out with tears and arguing and it was awful awful awful BUT it brought things to a head and we could then think about tackling it.
Also, whilst me and DH are together, there is still a similarity in what you describe about her DF telling you she eats plenty at his. My DH was very much in denial for a long time, describing it as disordered eating, part of her neurodivergence etc etc. Don't let that distract you from what may be the reality of an ED. Start the ball rolling with Drs etc as soon as you can. Keep pushing for appointments, quote the NICE guidelines (in the thread somewhere and in Eva M book) if you feel you're being fobbed off.
She won't hate you forever (I thought that too) but she's battling this thing in her head - there's DD, there's the teenager, and then there's the ED. Your DD is still in there but the fight and the swirling vortex of weird emotions in her head are so difficult for them to navigate.
The Eva M book and vids are SO SO helpful. There's a Caitlin Moran article/chapter from her book which I found helpful too, only in that I didn't feel so alone. And this thread.
Sending love and strength your way.