at this point,, get out the drill... use a 1/4" bit, drill straight into the torx head area,,,, when the bit clears where the head changes to threads, the head will snap off, and remain on the bit... stop drilling as aoo as the head breaks off...
after you have the derby cover off, you may attempt to remove the screw with a good pair of vice grips,, after hearing the screw to appx 400°F with a propane torch... or,,,,
If there isn't enough screw protruding,,, and depending on your skills with a drill,, (no offense, but by the trouble you've had with this, so far, I would say low),,, take the cover off and use a drill press to insure you drill a straight hole.. start at about 1/8" and go up incrementally until you reach the correct diameter hole for the screw extractor,,, remember to heat the screw, before attempting to remove it..
snapon makes a really great extractor set.. ... i highly recommend it, or you can buy just the size you need,,, snapon sells to the public through a web site, or over the phone...
The part # for the tool that extracts 1/4" screws is #E1 you will need a sharp drill bit in addition to this tool...