Bummer. You could try to Dremel a slot and apply some heat. Don't know what you've done already but if you can carefully pop the spring out of the frame without damaging the frame that would take a lot of the tension off it. Then it should come apart. Had a similar experience with my Uberti. Didn't need an impact tool just a good bit and a plastic mallet. I did wreck a screw on the heat shield on my Norinco 97 trench gun, penetrating oil, impact tool and a little luck got it out. Final thought would be to Dremel the spring inside of the frame where the screw is until the spring comes apart, you probably want a set of slixsprings and need new screws anyway. I would cut out some thin wood to tape or clamp on to protect the frame incase the tool slips.