how to tighten thread on the valve lash adjustment screw

Yeah 'lifter'... you know, that other word for 'rocker' LOL

Well, I doubt that your set screws will bust anything as I suspect you are not really torquing them down hard; they just need to stay in place to do what you want. Keeping the adjuster from moving might require more torque and that was my concern over the setscrew idea.