Well maybe you will have better luck in the future. I don't know why you're having so much problems as I don't believe there is any loctite or similar substance on the screw. I don't like the anti tamper screws but you should be able to do it. Back in May I did 2 in about 15 minutes including getting them out of the safe. One was a gun straight out of the box and the other I had 4 years.
You might try and see if the screw budges any tightening it a fraction. I'm sure it's possible someone tried to permanently fix it. You could also get a pointed flat blade screwdriver and tap it counter clockwise at the outer most edge of the screw.
This motorcycle image was the only one I could find off hand that kind of explains it.
