1. In a 2- to 3-quart pan over high heat, bring broth and 1 teaspoon olive oil to a boil. Stir in couscous, cover pan, and remove from heat. Let stand until broth is absorbed and couscous is tender to bite, about 5 minutes.
2. Meanwhile, in a 5- to 6-quart pan over medium-high heat, stir garlic in remaining teaspoon oil until it just begins to brown, 1 to 2 minutes. Add chili powder, coriander, cumin, and chili flakes; stir until fragrant, about 30 seconds. Add tomatoes (including juices) and bring to a boil over high heat. Stir in frozen vegetables and cook, stirring often, for 3 minutes. Add thawed shrimp and cook, stirring often, until shrimp are opaque but still moist-looking in center of thickest part (cut to test) and vegetables are hot, 4 to 5 minutes. Add salt to taste.
3. With a fork, fluff the couscous; spoon equal portions of it into wide, shallow bowls. Spoon shrimp and vegetable mixture, including juices, evenly over couscous and sprinkle with parsley.