Preheat oven to 350°. Line 20 muffin cups with paper liners. Combine the first 5 ingredients. Beat on low speed 30 seconds; beat on medium 2 minutes., Fill prepared cups half full. Bake until a toothpick inserted in center comes out clean, 15-20 minutes. Cool in pans 10 minutes before removing to wire racks to cool completely., For frosting, in a large bowl, combine 2 tablespoons lime juice, meringue powder and water. Whisk until frothy. Add confectioners' sugar, shortening, and remaining lime juice. Beat until smooth; frost cupcakes. If desired, garnish with lime zest and slices.