Removing a Schlage door knob with stripped screw heads
I'd drill the existing screw head off, use a bit slightly larger than the shaft of the screw but smaller than the hole in the knob. The stripped out screw should keep the bit fairly centered even when used at a slight angle. Once each head is removed you can pull the knob and try a screw extractor on what's left, but there's a good chance you'll need to replace the entire lockset at this point.