In a medium saucepan over medium-high heat, bring the water to a boil. Add the couscous in a stream. Stir once. Remove from the heat. Cover and let stand until the couscous is tender, 12 to 15 minutes. Mix in 2 tablespoons of the sugar. Cover tightly set aside.
Scrub one of the oranges under running water and pat dry. With a vegetable peeler, remove the zest, being careful not to peel away any of the white pith. Finely mince the zest. Set aside.
Squeeze the oranges to obtain about 3/4 cup of juice. Strain through a medium-meshed sieve into a small saucepan. Over medium heat, boil down the juice until it has reduced to about 2 tablespoons, and has the consistency of molten honey, 4 to 5 minutes. Stir in the liqueur. Set aside.
Peel the mango and cut the flesh away from the seed. Cut half into thin wedges and coarsely dice the rest. Set aside.
In chilled metal bowl, whip the cream until soft peaks form. Fold in the vanilla and the remaining sugar. Refrigerate 1 cup of the whipped cream for serving. In a medium bowl, combine the rest of the whipped cream with the yogurt and the diced mango. Refrigerate until ready to serve.
Before serving, combine the yogurt-mango mixture with the couscous. Spoon equal amounts into 6 parfait glasses or small bowls. Top each dessert with a generous dollop of the reserved whipped cream and mango wedges. Drizzle with orange sauce, sprinkle with minced orange zest, and serve.