I have ran into the same problem with a stubborn trigger plate screw that probably hadn't been removed since the Kaiser was a teenager. I made no progress after an extended penetrating oil soak until I figured out a trick.
I ground the tip of a cheap soldering iron to fit the slot tightly. Every afternoon I would set up the iron in the slot and let it sit for 15 minutes or so to heat the entire screw. Then put the oil back on it. As the screw cooled it pulled the oil down in around the threads. After a week or so it came out like it was installed yesterday.
