I honestly don't get the whole "honeymoon period" concept either. Granted, I have only had a few horses in my life, but in my limited experience I find the first rides with a new horse are the hardest for both horse and rider. We are both trying to feel each other out and establish trust in each other. And we are also trying to learn each other's body language and communication skills. So both of us may be a bit on edge. But then each ride I find we get better and better. More relaxed and more trusting in each other. Sometimes I think it takes a year to get to know a horse really well.
The rearing and striking sounds like serious disrespect in my book! I am so hard on my yearling, to the point of almost giving up on him, and he doesn't even do anything that dangerous, he's mainly just mouthy. So I don't know about that. She may be a serious alpha-type personality that takes someone very experienced to work with. Beyond that I won't comment because it is beyond my realm of expertise. Good luck to you and I hope you can get her sorted out.
PS. Could the trainer perhaps come out and help you with her at home? Since that is where she commonly reverts to naughtiness?