Something or Nothing?

After a long time without posting im hoping someone can give me some advice or suggestions regarding my two year old daughter.
Here goes...Right from the word go she has being the opposite of my eldest daughter, and I know that all children are different and we shouldnt compare but my DD behaviour often has me questioning(in despair) is this normal? She is a screamer and screams alot and for a long time>She bites, pinches,pulls hair,scratches,stomps on people and animals, has no fears and cannot be dissaplined, I say this because I feel like Ive tried everything with her and it means nothing, We have tried sticker charts, telling her off, ignoring her, crying(real and pretend)putting in room, taking things off her, gently explaining things to her, it all means nothing, we either get laughed at,spat at or she screams which can result in her either wanting me and only me or she will get the nearest thing to her and go out of her way to destroy it.She wants to eat all the time but eats only certain things, alot of things go in but the most goes in and get either spat out or chewed then spat out,Her sister is daily hurt with either scratching or hairpulling as well as the animals in our house. She screams if music is on loudish or if we sing to her or to ourselves.
I feel terrible writing all of this but am really worried as things are not improving, her speech is the only thing that I could say had improved yet I know that its behind for her age,
She has had her hearing checked as well as her vision, she is under a paediatrician for family allergies, he has refered her to a neurodevelopmentalist who we see in a fortnight, and he refered her to the Early intervention team who we are waiting to see and Plunket has also refered her to the Early intervention team as well as a speech thereapist.
I love my little girl to bits and somedays I think, ohh things havnt been so bad today but it is often short lived. Im tired of complete strangers shaking there heads at me or telling me that she just needs a good smack. I feel sorry for her as she seems so frustrated alot of the time, or when Im telling her off she can be smiling at me which makes me feel angry and sad for her. I knw it sounds like ive got a really naughty daughter but ive got to the point where it seems like there is more than naughty behaviour going on and want answers. Has anyone had anything similar or any advice.
I hope this makes sense

Thanks for the reply, I will look into it. She has been tested for allergies and intolerances in food and all came back fine.