Bring 2 3/4 cups water to boil in a small saucepan and stir in the quinoa and some salt. Return to boil, reduce heat to simmer, cover, and cook until water is absorbed, about 15 minutes. Turn off heat, place corn kernels on top of quinoa, cover pan, and let stand for 5 minutes. Stir corn into quinoa, remove from pan, and spread out on baking sheet to cool for about 20 minutes.
In a small bowl, whisk together lemon juice, cumin, chili powder, garlic, oil, and salt to taste; set aside.
Place cooled quinoa and corn, red pepper, cucumber, scallions, jalapeno, and cilantro in large bowl. Add 1/2 cup dressing (or more to taste) and salt to taste; mix until combined.
Place large lettuce leaves side by side in a circle around a large serving platter so the curly tops extend just beyond the platter rim. Mound quinoa salad in center.
Arrange tomato, avocado, and lime around quinoa on top of leaves. Sprinkle pumpkin seeds over salad. Serve immediately or hold briefly at room temperature.