To avoid the stone breaking, the slab should basically sit on a wooden top (e.g., 3/4" plywood) that's as big or almost as big as the slab. You can cover the edge for appearance. That would also serve as the top surface of the half wall.
To avoid torsion forces, you need to support the free corners. At the back wall, you can fasten a horizontal support board to the wall that the top rests on.
For the outer corner, That's a lot of weight and a lot of leverage. I wouldn't trust hidden braces fastened to the half wall, or even the half wall attachment to the floor or the half wall framing staying square and plumb. Use a leg that rests on the floor (the wooden top gives you something to fasten to). It should also be fastened to the floor, perhaps with a tie-back to the half wall. You could even make the half wall an "L", creating a cubby hole under the overhang. If that would eat too much of the length, you could use something like a wrought iron railing that would provide support but only consume a few inches (instead of a leg).
That will still leave a very heavy 4' span that needs support. Put some type of brace in the middle that fastens to the half wall and provides support to a point most of the way to the outside edge of the overhang. You can avoid knee interference with a right-angle or curved brace. But it should be something that is seriously strong and rigid, not something like the utility brackets used to put up garage shelving that sag under load.