Agree with BNT that from the look of those pix, it's a leg issue, not a foot prob to be changed.
As for hoof gunk, I wouldn't bother. The horse needs to be fed correctly & have well balanced nutrition, to *grow* healthier hooves. Putting stuff on the outside is no more effective than painting fingernails. The outer wall is already dead, so can't be made healthier. I don't know that particular one & why it may be so hard to come off, but I wouldn't be inclined to use detergents or anything to cut through it, as that will also take off the protective impervious layer on the hoof walls.