My husband, who is definitely not a horse person, but who is intelligent and observant (not to mention long-suffering when it comes to all matters barn and horse), made an interesting observation about the series of photos I posted about Miranda a few days ago. He said the three photos in a row of her eye as it changes looked somewhat like someone having an epileptic seizure, and that the eye is actually rolling back - what I had previously described as the horse going away or the eyes looking dead. It may be that pressure, of touch or presence, causes a sensory overload and triggers some sort of neurological storm - perhaps not true epilepsy but something analogous. This would go a long way to explain the odd combination of a horse that clearly wants to be friendly to people, and has never been abused, but is unable to abide their touch or presence and then becomes enraged by the whole thing - and why shouldn't she? Here she is attempting to engage with people - whom I think she actually likes - and when she does she experiences a loss of neurological control and perhaps even something that is like physical pain. No wonder she's confused, angry and even sad. It would also explain the odd and immediate return to normal horse behavior once the storm has passed.
This would also explain the odd combination of rage and being completely shut down in other circumstances - those are the only two ways she can deal with her experience. In a herd situation, since she is basically a submissive horse, she can't rage if pressured but can "go away" by shutting down. Similarly, when ridden or handled on the ground, she generally is very dull and even unresponsive to pressure, although recently under my daughter's care she had been waking up a bit and responding more like a normal horse.
It's clear that whatever this neurological problem is, the behavior problems, and associated neurological pathways - I almost think of it as a computer program running - are long-standing. Although we will never know for sure what is wrong, perhaps these thoughts will give my daughter some comfort. The horse's issues aren't training issues, really, they're built in somehow. The fact that my daughter was able to gradually desensitize her to touch and presence and actually work with her and achieve what she did is amazing to me, and a true testament to my daughter's sensitive and effective way of working with horses. That said, no amount of training would be guaranteed to prevent the neurologically-driven behavior from suddenly reappearing, which is very sad I think.
I'm probably over-thinking this because we're struggling with this so much, but it makes more sense to me now.