She could be exactly like mom's boss mare. Great with what she knows, relaxed in the normal routine of life. Make a good lead line pony for kids even. But put her outside her comfort zone, watch out. She has to be drugged at the vet because she is so nervous. Wide eyed, freaked out, panicked, shaking, trying to escape. Tried to leave her at the vet overnight two years ago (there was a stallion there and she had not come into heat yet and it was the end of April) to see if being near a stallion would get her to start cycling so she could be bred AI. She busted the stall apart and had sustained some mild injuries. She also runs around like an idiot screaming when you leave her in the pasture and are out of sight with anyone from the herd. But once something is a routine, she accepts it and it no longer bothers her. But drastic changes, being put in a stall with strange horses nearby with nothing familiar, she is a nightmare to leave unattended. This is how she has been for the 10 years that my mom has owned her. I have done a lot of work with her, but you can't stop some behaviors/reactions when they are in a stall or pasture and left to their own devices. New trailers are introduced, and once she has been worked with that trailer, she loads without any effort. Going for a trail ride is no problem, she knows the routine, scenic trailhead, tied to trailer while being groomed and tacked up as well as one of the pasture buddies along side her.
Make a routine, something predictable, that she expects. It sounds like her problem comes when someone is taken from their stall that is her trigger for going bonkers.
I wish you luck