Yes definitely get stricter with dishing out the treats, she wont really want to do things your way if shes getting the treats anyway by doing it hers.
As for the warning signs that's really really difficult for me to explain, one because the signs are so subtle sometimes and can go from 0-100 in what feels like less than a second but also because I've been reading Ollies body language for so long its instinctive to me, a bit like breathing, I do it but don't really have to think about it. That said its not really like breathing because I still do make mistakes sometimes. The other things are there isn't really one specific sign, poofed up feathers can mean more than one thing, contentment, they are cold, sick, making themselves appear bigger for self defence etc. Raised feathers with tail feathers spread out is sign of a really angry green cheek by which point its probably too late to avoid the bite, its the more subtle signs before that that you need to pay the biggest attention to.
The only real way you are going to learn is through experience I'm afraid and its the signs collectively that tell you the bigger part of the picture.
Ollie will sometimes start strutting about and surfing or tapping his beak on the surface he is stood on, whilst this isn't necessarily purely an aggressive behaviour, we tend to ignore him for a few minutes as he clearly isn't focused on his training, failure to leave him alone will usually lead to aggression because we aren't listening to him. We can usually resume training as soon as he has stopped strutting about with no issues.
"When I got her I was not expecting her to be so... difficult.

" oh yes, I can totally relate. Ollie was the first truly challenging bird I've ever had, the breeder said GCC can be very nippy. I kind of took the warning on board but thought "oh its just how their brought up, not many pet owners understand about parrots and train them" actually I will still stand by that but the journey it takes to get them to be decently behaved is a long one (particularly when your partner has his own ideas). I have never given up on a bird and never will but I will admit to thinking (on more than one occasion) that I had made the wrong decision and I will even go as far to admitting that there were fleeting moments where I did regret bringing him home. Top and bottom of it is that perseverance will get you through. Looking back now I cannot believe that I shed tears over the fact I couldn't do a thing with him without getting nipped (he was really that bad despite me not really doing anything wrong), I can't believe that I thought I had done the wrong thing bringing him home and I certainly can't believe I felt regret, I certainty do not feel or have any regret now. I'm proud of what I've achieved with him.