Is the screw mod not so reliable, given that it doesn't have a bolt holding on the opposite end?
Also, isn't it better to do all the screw and bolt modding from the bottom plate? When you are installing bolt mod, to prevent the hammers jumping, you have to put them in upside down anyway. That means the bottom plate will be facing up. It is much easier to insert and screw in the screws or bolts from the top aka bottom plate facing you.
You will drill the holes through the rivet shafts from the bottom, that is, the underside when the keyboard is assembled and in use. Obviously, most of the work is done with all these parts upside-down. A simple wooden frame is a huge help here.
But you want to drive the screws through from the top, that is, when you use the keyboard you see the screw heads. Turn that over, and assemble everything from the upside-down position, and jiggle the back plate onto the protruding screw shafts. Then spin on the nuts and you are done.
A screw mod would only include drilling partial holes through broken rivet shafts and letting the screw cut its own threads into the inner surface of the hole you just drilled.