Hi Confused. It's a mounting aid which disables the lateral springs so you can access the screw holes more easily. Pops out easily when twisted.
Hi Hilary, Radical and Beast brakes aren't compatible. Best bet is to get a 105mm brake with the new binding. Since Radical 2.0 stock has pretty much dried up in that size, I'd look at getting the successor binding, the ST Rotation.
Hi Tim, it can vary a bit based on your proportions and usage, but generally you will want to be in the 130-135 range for touring.
Hi Timothy, the available lengths are listed in the drop list above. Looks like all but the 125 at the moment.
Hi Matt, the spacer isn't necessary for most folks. If you're unusually large or aggressive it may take some stress off the pins. We'll have those in stock in the fall.