Just getting back to this thread after a break. Glad to hear that you have done some work with her on the ground. All you need is a long leadline and a rope halter, and maybe a dressage whip or long stick.
Listening to your description, I wonder is she has some pain issues. Being that stiff might be from something "out" in her hips or spine or ribs.
Just a thought.
I read that if a horse cants it's hind end toward the inside of the circle, most peole try to line up the front and back by pushing the hind out. But I was advised to bring the shoulders over to be infront of the straying hind end.
I use inside rein and inside leg to lift the sagging inside shoulder and recover a bend.
Once she becomes more supple and relaxed on the ground, try doing some walk circles counterbent. This helps get a horse to really step underthemselves.
When the horse goes around canted inside like you have said, they area avoiding the effort of stepping UNDER themselves . They will not reach very far forward with each canter step of the rear legs
YOu might have her trot over cavalletti to build up her abdominal muscles to help be able to bring legs straight under her own spine.