I know what you mean about the "kidney punch" You could tightened up the bolts so the top of the backrest could not pivot on you when you sat forward. BUT I found for myself, that when that was happening to me I needed to adjust the backrest farther back and actually keep my back against the backrest. I found that to be true also with the Corbin. But that is just me and we probably don't sit in the same slouched position.
My backrest adjusting procedure as explained to me from a sales rep years ago is to start out with the backrest way back and go for a ride for at least 15 minutes to get my body settled into position. Then as I ride, I will pull over and adjust the backrest forward in small increments and ride for another 10 minutes or so for a test.
The sales rep said many folks start out with the backrest too far forward.
Maybe this can help you as well...