First of all, some "valuable" items can actually be sold, so you should check whether you can sell an All-Restore Mystic Stone in one of the shops.
If that isn't possible, your only choice is to let your party get defeated in battle so that you can actually use up an All-Restore Mystic Stone. Since your party seems to be very strong, your best bet is to reach the most difficult optional dungeon. During my playthrough, I didn't check if the optional superboss there allows the use of All-Restore Mystic Stones (they usually don't in Exe Create games), but the random mobs there should be able to make short work of your party (on the highest difficulty setting) if you let them.
