The steel in the screws has been hardened - you will need a tiny Cobalt alloy drill bit to make pilot hole where you need one to be, then another cobalt drill to enlarge it to get a tap in. I've drilled Stainless Steel with a 2 "AA" cell 60rpm $4 screwdriver before (standing on tiptoes on roof to reach chimney flue liner & cap 40' off ground) so I know it has nothing to do with the drill, just the drill bit. Break the EZ-out off and I disclaim any advice was ever offered
You can also try a little percussion on it to relax the bite the corrosion has on the aluminum, straight on and 45° taps with a punch. Even if you don't see anything it will have weakend the rust weld bond or moved some aluminum.
I believe that is one job on mine that I simply cut the heads off flush and tore off the wrap utterly convinced notching the new wrap there & plenty of Vulkem will be the way it gets reinstalled...