how does she move in the trot? Is she short and choppy to start, head in the air? I'm only asking because I have a half arab mare and her shoulder is very similar to your mares. She very tense and upright through the shoulder, and it's hard for her not to lock her legs too straight. The last picture you uploaded it looks as if she is stepping really heavy on the inside leg, and maybe she would benefit from having the her front hooves trimmed in such a way where the toe was a bit shorter and the heel balanced so the the weight and shock are distributed more favorably. It's a work in progress for me to get her to extend her trot and round her hind and lift her back, and moving in a better way is actually quite exhausting for her. Have you tried side reins or any suppling exercises? I'm interested in what methods you are employing as I am going through the same thing. I think it's built into the Arabs physiology, it's kind of our achilles heel to get her to loosen up. I had a chiro out to evaluate and work on this problem too, and it helped a great deal in getting things jump-started.