To make dressing: In a pint jar, combine the garlic, gingerroot, lime juice, sugar, peanut oil, nuoc mam, and hot pepper flakes, if using. Shake well. Set aside.
To make the salad: In a medium saucepan over medium heat, bring the water, bouillon cube, and peanut oil to a boil. Add the couscous in a stream. Remove from the heat. Stir once. Cover and let stand until the couscous is tender, 12 to 15 minutes. Set aside to cool. Mix with half of the cilantro, half of the mint, and half of the peanuts.
In a large bowl, combine the carrots, cucumber, onions, cabbage, and remaining cilantro and mint. Add the dressing and toss thoroughly.
Spoon equal portions of couscous onto 6 dinner plates. Top with the vegetable mixture and the shrimp. Garnish with remaining peanuts, cilantro, and mint leaves. Serve at room temperature.
Cook's Note: To cook the shrimp, rinse them under running water. In a medium saucepan, bring 4 cups water and 2 bay leaves to a boil. Add the shrimp, and cook until they are just pink, 4 to 5 minutes. Drain and let cool. Discard the cooking liquid and bay leaves. Shell and devein the shrimp.