I submitted this post yesterday, but it did not appear for some reason:
Good to hear you got it out. The fact that you were able to remove it relatively quickly shows it wasn't terribly stuck to begin with. Had the fool that damaged it so badly simply used the same methods you did, he could have removed it with zero damage. Some people should never be allowed to touch a screwdriver.
Good luck finding an original replacement screw. 1883 Colt double production numbers were pretty low, only a few thousand, as I recall. I let a real nice one get away from me at a local farm auction about 20 years ago, and have been kicking myself ever since.
Opening up the countersink in the plate to remove the damage there would be best done with a milling machine. But then you'd need an abnormally large screw head for the replacement screw. So I'd leave it as is or repair the damage. Since you are near a large city, it might be easy to find a good TIG or laser welder who could repair the damaged countersink, and the screw head too. But it's probably easier to just have a new screw made if you can't find one.
There is a current Thread in the Double Rifle forum showing laser weld repair done on a Wilkes 10 bore double rifle. The cost for the welding, including shipping to Alaska, seemed very reasonable:
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=670630&page=1