As I was reading your post, I was thinking to myself "I could've written that five years ago". And then I saw your pony's pictures and think that perhaps our mares could be related! I'd love to see her pedigree.
I also bought a pony that was a bit misrepresented. She'd been off the trailer and in her stall for about 15 minutes when she put her head in the corner and tried to double barrel my head off of my shoulders. Couldn't touch her, not even to get a halter on her. She wanted nothing to do with us, even if we had food. Took me less than 24 hours before I put her up for sale, fully disclosing her wild streak. I'm soooo glad she didn't sell. She is my soul now.
It took at least two full years before she came around, but she did. In the beginning, we couldn't get near her, even in her 10X16 stall. We wanted to sweet talk her into liking us, and would try the less invasive things like sitting in her stall and talking, holding her feed, easing over to try to touch her. She would just explode, and she'd would have rather starved than get near us. Finally, after several months of this nonsense I'd had enough. Hubby came out with the calf rope and dropped a loop over her head. She was very offended by that but it didn't take long to teach her that "Face me" meant just that or the calf rope would come out and she'd be looped like some dirty little range calf. That's when the lightbulb went on for all of us and progress was pretty fast. This pony wanted a leader.
Once I could get a halter on her, I used T-Touch and round penning. And long walks together. Farrier work was a rodeo for the first year, and she still will give our farrier the hairy eyeball and do some dancing around before finally settling. But what really changed her was getting her ready for our first show (and yes, it did cross my mind that I was nuts trying to show this wild hare). I worked with her twice a day and introduced something new every couple days for a few months. That's when we really bonded and she learned to trust me. She loves to learn and work, and that was the key to her heart and mind.
For Sassy, the 'wild hare' act is now just that, an act. When you call her bluff, she is a very smart and willing pony. She is still shy with strangers, and still has her "The Sky is Falling!!!" moments, but is very handlable and sweet.
I'm glad that you are going to stick with this pony. I bet dollars to donuts that she will end up being your favorite.
Here's my wild girl, Sassy. Erase the white, and I think she and your mare look a lot alike in the face.