That adjustable slack washer is the Achilles heel of the hub. My grub screw has gradually worked itself to a setting with more slack and needs the occasional tightening to get it back into position. Also the hole threaded for the grub screw creates a stress point for it to crack around the screw.
the self loosening is part of my worry about the hub - in an ideal world the slack adjuster sits perfectly well at its setting and there's no wiggle to work the screw loose etc, but it's always limited to manufacture tolerances i guess. i wonder if you could just pretend that it's metal and secure it with loctite (give it overnight to cure properly!) would work on such materials.
a second ghetto fix would be to add ANOTHER grub screw into the hole, tightening it against the first one (essentially treating them like cones on older generation hubs' conenuts) then cutting/ filing off the protruding excess. then against, putting a few actual spacers on is much less effort than this.
as for stress concentration, that part shouldn't be seeing any stress, since the whole slack adjuster is only something for the clutch mechanism to rest on, at the force of the clutch spring/drag mechanism spring.