When i was a teen my mum ran a helpline called "Parents in Crisis" which was for the parents and family of ADD/ADHD children.
ADD/ADHD shows strong family association, but they truly don't know how much of it is genetic and how much is parenting. The consensus seems to be that it's a mixture.
Getting a diagnosis is important but it's also important to remember that with things like ADD/ADHD they cannot tell the difference between "true" ADD/ADHD (a neurological thing without outside influence) and ADD/ADHD behaviours, so having a diagnosis that she has one or both conditions isn't a life sentence and a lot can be done, especially when kids are young (some of our parents had kids in their teens and not yet diagnosed, heading towards jail). So diet IS ABSOLUTELY worth looking at, if it's a neurological thing it won't help but it won't hurt and if the behaviours are being exacerbated by something she's allergic to it will make a potentially big difference to her.
Two things that my mum noted through her interaction over 7 years with these kids are 1) you cannot teach an ADD/ADHD child not to hit by hitting them - i'm not criticising your methods, i have a normal child who i frequently want to strangle! - it's just what they found. Just as she links the simplicity of action "well my sister was doing it" when her sister is only 2 and she is 5 and should know better, she will NEVER get that mum and dad can hit but she can't, and 2) after they began funding parents to go on parenting courses 100% (there were only about 14 families/year) of them found their kids began to behave better and around 80% of those whose parents changed techniques actually had their diagnosis reversed, so great was the improvement. In almost all of the situations there was at least one parent with ADD/ADHD too, and the more parenting classes that parent did, the faster the situation improved. Don't misunderstand me, there were SOME children with ADD/ADHD and parenting could only somewhat limit the behaviours but there were MANY more where the parent-child dynamic had gotten into a place where neither were able to change and things were spiralling downwards. Parenting techniques cannot cure true ADD/ADHD, but they CAN allow you to parent in ways which will minimise it, and in ways YOU choose, rather than are driven to in desperation. With ADD/ADHD staying dead flat calm can be helpful, as the child cannot aprehend their own emotions regarding what is happening, and if the emotions of those around them also run higher they start to get out of control. If you remain calm (which is such hard work NO one can do it all the time) they have only their own emotional response to deal with, and not those around them too. I have many times watched my mum state the fact "it's time to go home now" and then as the child kicked and screamed and scratched and bit, calmly lead them away, saying "now we are going home". It takes a lot of work and the results are small but calmness is contagious after a while. Mum's silence gave them a chance to reflect.
I feel for you hun, it is so hard to raise a difficult child, but as you see in your
DH, they can grow up to be truly wonderful people. You are up against it now, but you're actively seeking answers and you WILL find your way through this, all of you.
I would concentrate just now on getting a formal diagnosis and looking at Triple P or whatever is local to you to let you fill your parenting toolbox up with ways to help your
DD. Hang in there, you can do this!
Bx