Cut 2 heart shapes from each cake slice, using a 1 1/2-inch heart-shaped cookie cutter.
Reserve excess cake for another use.
Cook jam and 1 1/2 tablespoons water in a small saucepan over medium-low heat, stirring until smooth. Remove from heat, and let cool slightly. Place cake hearts on a wire rack in a wax paper-lined 15- x 10-inch jellyroll pan. Brush jam mixture evenly over tops and sides of cake hearts.
Knead marzipan gently; place between 2 sheets of plastic wrap, and roll to 1/16-inch thickness. Remove plastic wrap, and cut out 28 hearts, using a 1 1/2-inch heart-shaped cookie cutter. Place marzipan hearts over cake hearts. Cover and chill 1 hour.
Combine powdered sugar, 1/2 cup water, and corn syrup in a large saucepan; cook, stirring constantly, over low heat until mixture is translucent. Add remaining water as needed, 1 tablespoon at a time, until glazing consistency. Stir in red liquid food coloring until blended.
Spoon icing evenly over cake hearts, coating completely.