Stop right now. Put away the screwdrivers. Get some penetrating oil. Real penetrating oil, not WD40. Kroil is the best, but is hard to find. Spray some penetrating oil on the joint, and spray some inside the receiver as I think that screw goes all the way through into the receiver (not a blind hole). Give the screwhead a few light taps with a hammer - you want a little shock and vibration on the threads to help the penetrating oil. Now put it somewhere nice and warm for 24 hours.
Time is your friend. After 24 hours, warm it up with a heat gun or blow dryer, NOT a torch. Remember, the idea is to heat the receiver to expand it, rather than heating the screw.
Firmly place the receiver in a vise with protected jaws - wood, leather, plastic. Make sure your screw driver fits the slot well. Now apply torque slowly to feel for any movement. If it keeps moves freely, continue on unscrewing it. However, if it moves say 1 mm and you feel high resistance, stop. Add a little more penet. oil and tighten it back up. Now gently twist it back and forth across that 1mm of free movement, then 2 mm, then 3 mm, until it frees up completely. Many times I've loosened a screw or bolt, but then the rust on the threads tightens it back up again and the bolt shears off. But twisting it back and forth until it turns freely is the trick.
