Zest 5 oranges using fine side of a box grater or a razor-sharp grater, such as a Microplane; set zest aside. With a serrated knife, cut ends off all oranges. Set 1 orange cut side down on a cutting board. Following the curve of the fruit, cut off peel and white pith. Working over a strainer set in a bowl, slice next to each thin white membrane into the center of the orange, releasing segments of orange into strainer and leaving membranes behind. Squeeze juice from membranes into strainer and discard them. Repeat with remaining oranges until you have 4 cups segments and 1/2 cup juice (save remainder of both for other uses); put in separate bowls.
Add zest and 1/2 cup liqueur to bowl of juice and set aside.
In a stand mixer, combine mascarpone and cream and whisk until stiff peaks form. Add 1/2 cup marmalade and whisk to blend.
Arrange a third of the pound cake slices evenly in the bottom of a 3-qt. trifle bowl or other straight-sided bowl. Using a pastry brush, soak cake with 1/4 cup liqueur mixture. Dollop a third of the marmalade cream on top of cake. Arrange 1 cup orange segments over cream, making sure widest part of segments lies against the side of the bowl. Repeat with 1 more layer of cake, pressing down on it slightly before layering on same amounts of liqueur mixture, cream, and oranges. Arrange last layer of cake, drizzle with remaining liqueur mixture, smooth remaining third of the cream level over cake, and arrange remaining 2 cups oranges in a decorative spiral.
In a microwave-safe bowl, heat remaining 1/4 cup marmalade with remaining 1 tablespoons liqueur until hot; stir to combine. Brush warm marmalade over orange slices. Chill until ready to serve.