Whoa, love the diagram! It's very helpful in explaining what you mean. I agree she's sitting too far back, but IMHO it's because the saddle is cantle-low (due to being too far forward and in need of a riser pad/shim) and her stirrups being way too short. With the cantle low and saddle forward on the wither there's really no way she can sit in the designed balance point of the saddle-- because that spot isn't where it should be and would put her out of balance if she sat there (tipped back and in front of the motion). In these situations the body naturally seeks the security of the deepest part of the saddle and the point closest over the horse's center of balance, hence sitting too far back in the saddle. You can see if the saddle were shifted back and shimmed it'd be below where she's sitting now, which is roughly correct in relation to the horse, despite it being incorrect in relation to the saddle. Remember too that Dressage saddles have high cantles when compared to close contact, so if she were sitting in the correct place-- closer to the pommel, as you helpfully illustrated-- she wouldn't have extra seat, and in fact I'd be concerned her delicate bits might be uncomfortably close to the pommel. You can look at where the flap ends in relation to the cantle for an idea of just how much of the cantle should be behind her-- keeping in mind her leg should hang down from the hip and be in the center of the flap. You'll also notice her upper body is close to vertical, so if she forced open her hip angle she'd be behind the vertical and behind the motion, and farther away from being over the irons. To open her hip angle she needs to put the sale in the correct place and use a shim, and lengthen her stirrups to let her leg come back, thereby opening her hip joint.

In addition to that her too short stirrups push her body back in the saddle because there's literally not enough room for her lower body between the stirrups and the correct seat of the saddle. The only way she can fit herself in that space is to sit back on the saddle. This naturally forces her leg in front of her causing a "wedge" effect where the body is unable to get security through being in natural vertical balance over the stirrup and instead "wedges" itself in more horizontally between the stirrups in front and the cantle in back. It's actually a very secure seat-- called a safety seat-- and one every rider should have in their "toolbox" for emergency situations where the horse is threatening to explode, for the very reason that it locks the rider into the saddle. However it's obviously unsuitable for general riding.