Crikey, i had the same problem when I did one of my windows. Thankfully mine eventually came out.
I would try and part the screw using heat to start with. If you get a soldering iron on it and heat it up, and then cool it down quickly that could help break the galling. You could also try a punch on it or impact driver. As you say, you don't want to damage the frame. To get the purchase on it you need I'd cut a slot in it with a Dremel, get out my socket set and proceed very, very carefully.
Maddening when such a simple job turns into something so involved. Good luck!