You may be able to slightly reduce the density of the Sharpie marks with isopropyl alcohol on a cotton swab. My experience, however, is that *at best* this will only very slightly reduce the marker.
Worth a try if you'll have to remake the mask anyway if you can't remove it, is to get "non-chlorinated brake cleaner spray" at the local parts store. It's made to remove oil from brake drums so the brake shoes don't grab and slip randomly, but it will take permanent marker off steel, paint (though it can also lift the paint), and some plastics. What it would do to gelatin I can't say -- but if the alternative is remaking the mask, it's worth a try. PLEASE use it outdoors -- your liver and kidneys will thank you!