Yrs, Okan, you cannot touch the sole. It is very thin from the bone inside in the wrong position. Where loosie put the red lines don't touch!!! This is protection for the bone. In the back, as loosie says, careful, but it has to come down to put the bone in the right place. This is why we say cut only the back every 10 days or so to bring the bone in the right position. Little at a time so the tendon who is attached to the bone does not stretch too much.
To cut the walls, nailman can do this from above instead of underneath easier. Especially at the toe. Go from loosie's blue line up and cut/rasp in a 45 degree angle.
And yes she will have to carry her weight on the whole sole, not on the walls.
This is very important, because when the wall carries, the force of her weight will pull the nails away from the bone even more.
I know it is hard to see her down so much and it can take time until she really gets better. It takes time because the nails have to grow down healthy, 1 cm a month.
You are on the beach, right? Can you make her her own beach in her stall maybe? Put at least 15cm of sand on the floor instead of hay? That will make it easier and less painful for her to stand. And when she gets better a bit, give her also some hard ground to stand and move(it helps to get the blood circulating in the foot=growth).
Losing the weight is good, but she needs proper nutrition. Can you find a vitamin/mineral supplement for horses and give her this together with the biotin and a little horsefeed?