MM, I think I understand a little what you're dealing with. I occasionally groundwork a spoiled rotten QH gelding for some elderly neighbors. These people are only his second home in ~15 years, and they've had him for about 8 years now. He ruled the roost at his first home (my understanding is that he was never exactly weaned until he was sold), and rules the roost now. The owners simply aren't physically able to provide the leadership that he needs to be a good citizen, and honestly think that his semi-dangerous high-jinks are "cute." :roll:
This gelding basically has a very low threshold of tolerance for human direction. There just comes a point where if you assert yourself, he gives you the finger and throws a tantrum until he breaks something and gets his way. He is a master of "shock and awe" tactics. He's better for me than for the owners, because I've managed up to now to not push him to a tantrum while still getting my point across and not letting him "win." They and his previous owners simply allowed a pattern of behavior and dominance to start, and now, 10 years later, we have a "problem-child."
The only advice I can give you based on personal experience is to be creative, be totally and completely unemotional about everything, and be satisfied with little steps. The gelding above responds very well when I adopt the general mindset of taking his ideas to my defined extreme; he wants to do x, and I'll encourage that movement until he's bored with it and 5 seconds longer, then let him rest on my terms.
I'm sure that you already know how to do all of this; Jynx is your horse, you know her and her mind better than anyone. I'm just offering up my similar experience, hopefully you can draw some use from it for yourself. I'll be lurking to see if there are any good suggestions for my neighbor's gelding as well.
Best of luck!!