Thanks everyone for your responses.
I ended up drilling a slot in the screw using a reinforced Dremel cutting wheel, ruining the factory sight in the process (the wheel was too large to avoid cutting into the sight itself). No love lost there. I heated up the screw with a small butane torch and after hauling on the screwdriver it finally broke loose. There was some sort of opaque non-coloured adhesive residue on the threads. A hearty thanks to Ruger for wasting mine and many other people's time on this.
As for the warranty claim, I did hear back from Location Snap Shots, and they said the removal would be covered under warranty if I wanted to send the rifle in.