Get easy out, about £15, it drills into the screw, and has a left hand thread. It'll come straight out.
For future reference, removal is best done with a impact screwdriver.
Not sure on the replacement part, but someone will know. Its very rare for cars to still have them once they've had a few disc changes, as the only job those screws fulfill is to line up the disc and hub for easier wheel fitment, they are not a safety item.