I know what you mean about the disc retainer screws. I had some very stubborn Mini ones recently. I applied penetrating fluid to soak, then used a Makita cordless impact driver (not an impact wrench, a driver for wood screws etc) repeatedly reversing the direction. As soon as some movement showed more fluid was added and soaked and the process continued. Took best part of 15mins to get one to come out. The impact driver meant the Torx screw kept its socket without rounding out, but it was a close thing. Those screws projected through the hub, which was half the problem, the exposed end was badly corroded and I oiled them to help getting it back through the thread.