Haha you are still hung up on that knock sensor deal eh? Don't worry about it dude. As long as you aren't constantly driving WOT at a billion rpm, you have nearly nothing to worry about using 91 to cruise around.
That system isn't mine, its my Dads. Its pretty old, and apparently isn't made any longer. I believe the company that used to make it got bought out. Anyways, its a standalone system, so yes, it can monitor alongside the OEM equipment. You could set how sharp you want it to pick up sounds, and it would register on an analog gauge that is affixed to the control box. IIRC, the instructions said to bolt the actual sensor as high as possible on the block. However, I would wager a good bet that the OEM system is more accurate because of carefully calculated placement (not just randomly stuck on the side anywhere) and also it is calibrated to know what sounds to pick up. This standalone system is just a guide, and after all, is just only a needle with a gauge. A cylinder pressure sensor would probably be a better bet of seeing whats going on inside.