This is an interesting thread to me, because my horse (quarter-Arab) has similar problems, and I think your Arab's progress is terrific!
ALL the instructors I've had with her say it's okay, at this time, that the head's too low, even when I point out she's on the forehand. "Just push her out a little more," they'd say. Because while in this "incorrect" position her back is learning to stretch, not an easy thing for this type of horse.
I once had a lesson with a substitute horse, as my horse was lame. I rode a First Level (about) Hannoverian mare. I told the instructor what I was working at with my own horse; but this horse was so different, my home problems just didn't exist. You can't start horses all the same way.
When my horse occasionally goes behind the bit, I raise my hands. Why? Because I watched Buck Brannerman school a horse like that once, and I often try out different methods, and in this case, with my horse, the action was pretty gentle (on the lips instead of against the bars of her mouth) and for some reason my horse understood it meant her head was too low. Strangely, it lets her know when her head is too high, too.