Why wouldn't you finish cutting through to make a slot? The slot has the advantage that using the axe fixes the axe head more firmly into the slot by wedging itself. Even if you wedged the stone in the pocket, the pocket will loosen and the stone will eventually bottom out and won't be wedged anymore.
> Why wouldn't you finish cutting through to make a slot?
Because then using the axe would quickly force the stone through the handle, as the only restraining force will be grip from the sides. That force on the sides will also encourage the handle to split. With it in a cup, the force will be on the back.
I'm skeptical that plant glue alone would work without additional lashing. Its brittleness is used as an intentional feature in hunting weapons so that the small blades detach and stay lodged in the prey.
i don't see why it matters whether the center of moment is within the handle or not; the problem is that the glue has to be able to resist the moment without breaking, but it's much closer to the center of moment than the blade edge
say the blade is 100 mm long and 30 mm thick with a cylindrical back edge; the moment is being applied to the blade edge 85 mm from the center of moment, while the glue is resisting the moment from only 15 mm away, giving a 6.7:1 mechanical advantage, and the force is the full force being applied to the workpiece multiplied by sin θ, where θ is the angle the central axis of the blade makes with the force applied by the workpiece on the blade; typically this will be on the order of 0.1, but in a bad strike perhaps closer to 0.3