PixieGrace not read all comments but trying to read all your comments.
My dd was similar, she has long hair, very matted.
MY advice:
Get rid of nits, that is a must or the phone/internet whatever goes, turn it off when dh is out of house if he won't support you.
Let her be responsible for brushing it herself, if it is matted, so be it, offer to cut the matted out if she wants. Maybe warn her she may be bullied for matted, dirty or badly cut hair - be prepared for her to say that you are bullying her for saying this (my dd did!).
My dd now has a 'boyfriend' and has suddently taken an interest in her hair. It looks beautiful, she is 12 too and have had about 8 years of this!
I'd personally insist in a trim twice a year, just to tidy the ends up. I would not insist on her having it cut, it's not really fair on her IMHO. You will need to warn the hairdresser and may need to pay more for her to have it trimmed as the hair dresser will need to get the comb through it.
You do realise, I hope, your real problem is not your dd's hair but your nasty husband?
"DH gets angry with me if she tantrums if I make her do something! Feel like I'm between a rock and a hard place!" You are.
"Well he's just had a massive go at me for suggesting I wash her hair, and saying he doesn't want to hear my voice as it's too loud! Everything gets turned back onto me!"
He sounds fucking horrible, what are his good points? Any?
I also wonder if your dd has some unknown special needs, ASD? Just curious, mine does.