The HV strategy was as follows:
Food is to be placed in front of DD. She then eats what she wants from it. She gets no food-related attention ie we're not allowed to tell her to eat up, start eating or even to try it. Just ignore any behaviour. When we've finished, she gets a 5-minute warning that the table is about to be cleared.
This HAS helped us sort out the "behaviour" at mealtimes. Before she used to end up face down on the floor screaming because she didn't want to eat whatever was given to her. Now she will sit quietly and will even tolerate seeing veg on her plate, even though she doesn't touch it and won't eat anything which has been touching it! But she doesn't always eat something and often goes to bed without having eaten anything at all.
What hasn't been sorted is the actual "eating" which I have been told will come later and that we can't actually force her to eat, we can only control the behaviour. To be honest I'm banking on school sorting out that issue as she'll be having school dinners and hopefully seeing her friends eat will help. The HV says it takes ages for children to become vitamin deficient and we only worry because we're "food aware".
DD is extremely willful, headstrong, clever and controlling. I suppose I should be glad this bout of dreadful behaviour is a phase, which I am sure it is.
I agree that upping the ante is counter-productive and she's got herself into a cycle of behaviour. Sometimes in calm moments she says she doesn't know why she does certain things. I think when your authority is so obviously being challenged it's easy to not want to be seen to be giving in, especially when it can lead to all sorts of issues. But I think we will sort out the big and small issues and start ignoring the challenges over small issues.
She has plenty of quiet time to play in her room. So it's not that she wants that. She clearly wants the attention and seems to be craving the bad attention. She gets so much positive attention that I think she's bored of it and wants to spice up her day some more.