My brakes are due right now, but haven't got to it yet. I know on many cars with rear disc brakes, the emergency brake doesn't use the caliper itself, but actually has a small drum brake type setup in addition to the rear disc brake equipment. This small drum brake is what functions when you apply the parking brake (haven't checked mine out, so I can't say this for sure, but something worth checking into). I would imagine forcing the piston into the caliper improperly probably dislocated or bent/broke something inside the caliper assembly since it seems the piston is no longer moving in a 90 degree angle to the rotor surface. As far as the screws you mentioned, they ARE a PITA sometimes because of heating up/ cooling off and corrosion; I've destroyed many of them and actually am missing half of them from the front end of my car. From what I understand, these screws are there to hold the rotor in place during the assembly process of the car and will not cause any issue if not there - many cars don't have them at all. I don't see any way possible that those small screws could bear the pressure that would be involved in the braking process (they're so weak and soft that you can't even get them out without stripping the head in many cases) - the pressure is transferred from the wheel to the lug studs to the rotor.