I think I'd start her up close, with a fetcher, not a catcher. No focus at all on control. Maybe even tossing into a net. I'd find whatever effort/distance that she could throw from and maintain posture. I'd give her a cue of rolling her back foot up onto the toe in a relaxed fashion. I'd ask her where on her arm she felt brush and talk about how posture helps brush. If it helped, I might ask her to try dragging her toenails.
Move back from there with more effort/distance until things break down. At that point, she might try a few dry throws to see if she can get the motion down, then try a few with a ball. She could also try throwing with her eyes closed.
Progress into walk ins, then full motion. In full motion, she might try an effort to drive squarely towards the target, but allow the torso to open naturally as the limbs extend. Toenail dragging might help. She might also try to imagine having an eyeball on her laces and think about the eye seeing the ground or the target. It might start feeling/looking a little forced at first, but eventually she'll want that back leg to come along for the ride, but in more of a toe down orientation which tends to help posture and get the hip in position for brush
Double check everything in slow mo. Work towards building her awareness so she can quickly identify and adjust on her own.
Here's an IR progression you might progress through:
Thanks a lot for this. We were probably trying to move to fast.