Combine first 3 ingredients in heavy small skillet. Toast over medium heat until fragrant, shaking skillet occasionally, about 3 minutes. Grind spice mixture in spice grinder. Transfer to bowl; mix in 1 teaspoon salt.
Whisk mayonnaise, lemon juice, chives, and half of garlic in small bowl. (Can be made 2 days ahead. Store spice mixture airtight at room temperature. Cover and refrigerate lemon aioli.)
Heat 2 tablespoons oil in heavy large skillet over medium-high heat. Add carrot and onion and sauté until crisp-tender, about 3 minutes. Add thyme and remaining garlic and sauté 1 minute. Add 2 1/4 cups water and remaining 1/2 teaspoon salt; bring to boil. Add sugar snap peas and cook until crisp-tender, about 1 minute. Using slotted spoon, transfer peas to plate. Immediately add couscous to water in skillet and stir to combine. Cover and remove from heat. Let stand 5 minutes. Fluff couscous with fork. Transfer to bowl and cool completely. Mix in sugar snap peas and dill. Season to taste with pepper. (Couscous can be made 2 hours ahead. Let stand at room temperature.)
Prepare barbecue (high heat). Coat tuna with 6 tablespoons oil. Sprinkle with spice mixture. Grill tuna until seared outside and rare in center, about 4 minutes per side. Refrigerate uncovered for 1 hour, then cut into 1/2-inch-thick slices. Spoon couscous onto plates. Top with tuna and drizzle with lemon aioli.
