Thank you, Dr. P!
I think another adult that she trusts telling her that eating food and using manners are important made a huge difference. We also did calm breathing as a family and I promised not to punish her if she slipped up and did something wrong. The huge difference in my thinking isn't that she did everything perfectly, but that she TRIED. Nothing I've done up to this point has convinced her to even try. When I sat with her and nagged her, she'd make a show of using her fork and chewing with her mouth closed. As soon as my back was turned or if I stepped out of the room, she'd smack, get her fingers in her plate, take either huge or tiny bites, try to eat as little as possible or not at all... the list of shenanigans has been endless.
Having an adult acknowledge her terror of [punishment/noise/everything/life] and offer solutions was enough to get her to listen to the idea that her body NEEDS that food, and that eating nicely might be a good idea so other people don't think she's gross. Hmmm, good thinking!
I love that she has someone so wise in her corner. :)
The deep breathing didn't do me any harm either. I've become so tense at mealtimes, dreading the inevitable fight, that I'm sure I exacerbate the problem. It really helped to have a success tonight. I hope it's repeatable!