1. Cook frozen edamame in a large saucepan of boiling water until just tender, about 4 minutes. Transfer edamame with a slotted spoon to a bowl of ice and cold water to stop cooking. When cool, drain edamame and pat dry. Transfer to a bowl.
2. Bring water back to a boil and cook shrimp until pink and just cooked through, about 3 minutes. Drain shrimp in a colander and rinse with cold water until cooled. Peel and devein shrimp. Toss shrimp in bowl with edamame, 1 tablespoon juice, and salt and pepper to taste.
3. Trim ends from grapefruit. Standing fruit on 1 cut end, trim peel and white pith away from sides with a sharp knife. Then, holding grapefruit over a bowl to catch juice, cut segments free from membranes and then cut each segment into thirds.
4. Halve avocado lengthwise, pit, score flesh with knife crosswise, then lengthwise, and scoop flesh out with a spoon into a bowl. Gently toss with remaining 1 tablespoon juice, mayonnaise, chopped herbs, and 2 tablespoons juice from cut grapefruit. Season with salt and pepper to taste.
5. If desired, set a 3-inch metal ring off-center on a plate and layer shrimp, avocado, grapefruit, and edamame, pressing gently on top to pack mixture. Carefully remove ring (If some of the layers fall apart, that's okay) and arrange lettuce around salad. Or gently toss shrimp, avocado, grapefruit, and edamame together in a bowl and arrange in layers on a plate along with lettuce. Garnish with mint leaves.