broken screw

Hi, I'm working on a Gruen 420SS movement and have broken the click screw on the barrel bridge. I'm not a pro (obviously!!) so I don't have all the tools. I've seen threads about using Alum but I'm concerned that the internal movement plated might be regular steel instead of stainless steel...

I just discovered that one of my case back screws has sheared off & now I'm looking to find out if anyone has any ideas on extracting the broken piece. It's broken off below the surface, so there's nothing to grab. Obviously, the water resistance is gone until I can get this fixed. Anybody...