Are you sure? Why on Earth would the film makers do a risky base-jump from the actual top of the building, more than likely there was a crash mat 3 inches or so below the top of the 'building'...
Its true. I can't remember the exact name, but its a special device that slows the person down before they hit the floor. This allows the to fall at a 'normal' speed, and makes for a more realistic scene. It says so on the commentary on the DVD
Yes, on the commentary the directors mention this device. Its a free fall device. Like bungee jumping. Actually its two cords attached to a device around the actors waste under his clothes, which slows him down only seconds before he would hit the ground. There was no mat used, to my knowledge. And when he's going up the side of the Guggenheim, thats a crane lifting him up the wall. All this is edited out but you can still see shadows.