I think that the only answer to getting the rear disc retaining screw out is IMPACT SCREWDRIVER. I've had one of them for years and was scared to use it on anything, but when the seized disc retaining screw on someone elses car would not come out - out from the depths of an old tool box came the impact screwdriver - and it worked a treat.
One thing to remember here is, the worst thing that can happen if you end up using the impact screwdriver the wrong way (direction) , is that you shear the head off the screw. So what, that screw is only there to save you a lot of trouble when changing wheels, my old B5 Passat does not have disc retaining screws, so VW provided a plastic threaded bar to fit instead of a wheel bolt when changing a wheel - it keeps the disc in line on the hub (so that you can fit the wheel bolts and when its at the highest hole, you can hang the wheel from it - quite handy really, I've since bought one for my wife's Polo 9N and an Audi aluminium one for keeping in the garage beside the wheel changing stuff - the Audi one is about 4 inches long though!