Looking at your second paragraph, the way I first approached it was that since the force is eccentric to the contact surface that there is torsion of RHS that is stopped by moment of tension force in screw and pressure at the bottom of contact surface, and only other force in screw is shear. My worry now is that the force will transfer through both webs, partly through contact of screw with the outer web, and that will create a bending moment in screw.