Good question. I have had the wad crack on these USA cases but the aluminium never corroded. I think I would inert your samples.
Carefully remove the wad by cutting around the circumference. You may need to make another cork one and paint it red if too brittle.
Remove any internal wads and very carefully drill by hand the flare cylinder using 6mm, 12mm drills and larger about 6mm deep at a time and empty out any compound and brush off the drill bit. When thin you can peel the liner away from the case with long nose-bent pliers. The flare dust will not ignite if you go slowly and remove any drilling dust.
Wrap the case in cardboard and tape, plastic wrap to protect the print on the case before you start. I have inerted 20 flares with no problems in all colours.
Illumination is the worst one for going bad and the hardest compound to remove.
I would use hydrochloric acid on a cotton bud to clean the primer corrosion. Rub it for 10 seconds then wash off with warm water. Polish with 0000 steel wool. Keep off the aluminium. Neutralise the area with sodium bicarbonate (baking powder) solution, wash and dry with a hair dryer.
I use Wattyl brass /copper varnish to paint the cleaned primer and inside the cleaned case.
If you spray the outside of the case, do very fine coat over the print and let it dry. If too heavy the lettering will run and spoil. Then do another heavier coat of varnish.
I hope it is OK to mention my inerting method. If not please modify or delete. Ron.