PB Buster is another good penetrating oil for such occasions. Once the old screw is out and you're ready to install a new one I recommend apply string wax on the screw threads then running it into the hole. This will act like a thread locker but doesn't seize up like your current situation and won't cause damage to the threads in the hole.
2008 Martin Firecat Pro-X (29" DL, 66# DW), home-made TRG, new VEMs, DS Advantage/HHA Pro-5519 front sight, No-Peep Sight Eliminator, Limb-Driver arrow rest, B-Stinger stabilizer/dampener, custom upper STS, G5 Head-Loc quiver on custom mounting bracket. Gold Tip Velocity XT300 arrows with Bi-Delta 2.5" Sharkstooth vanes and Grizzly single-bevel broadheads, 498 grains at 262 fps (22% FOC)