The dust shields that fit on the back side of the wheel are called 'rotor-eaters' because they cut down too much on the cooling air for the rotors and thus you end up with warped rotors.
As far as the black dust, the Performance Friction pads from auto zone also release quite a bit of dust. My original pads from GM on my vette did not dust the wheels as badly. However, the car does stop ASAP. I just have to clean my wheels every week. (EVERY week).
No, aftermarket pads, by their nature, do not contribute to loud, squealy brakes. The dealer has given you a bum steer on that point.