After letting the offending screw sit with Kroil for a few days, with the gun in an appropriate vise, using the appropriate gun smith screw driver, apply the screw driver to the slot, rap on the end of the screw driver handle a couple of times smartly with a plastic/nylon hammer to loosen the screw.
If the screw does not turn with appropriate pressure, do not force it...if you snap one side of the screw head off, you are going to have to drill out the rest of the screw.
If, after the above procedures, the screw still does not want to come out, it has likely been red-loctited, and I would recommend just leaving it alone. Put the gun back together, and use it the way it is. Because, it is not broken...do not continue to try to fix it. If the day comes where you have to remove those screws, apply some heat to soften the loctite...consult with a gun smith/Ruger first.