Tell us about yourself!
If all else fails, and I really mean ALL, grind the whole head off the screw so you can get the cover plate off (or whatever you want to shift). Use a good quality HSS drill to drill into the aluminium casing all around what has now become a stud. This will allow you to remove it, but leaving a big, untidy hole. Fill the hole with good quality 2-part epoxy, as it starts to go tacky, screw the new screw in, let it set for about 12 hours - you should be able to unscrew it then, leaving a nice neat threaded hole. Leave it to set for 3 days. This will work only if you don't need to undo the screw very often, AND ONLY if it's not load-bearing, and it is a last resort. I did this to replace a seized broken carb mounting stud on a 1953 250cc BMW.