Arrange 2 lettuce leaves on each of 4 plates. Combine lime juice and next 6 ingredients (lime juice through pepper) in a medium bowl, stirring well with a whisk. Add mango, onion, and cilantro to juice mixture; toss gently to coat. Spoon mango mixture evenly onto lettuce leaves.
Drizzle scallops with 1 tablespoon lime juice; sprinkle with 1/4 teaspoon pepper and 1/8 teaspoon salt. Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add scallops; cook 4 minutes on each side or until lightly browned. Arrange scallops evenly over each serving.