Combine shrimp, 1/4 cup lemon juice, reserved lemon halves, parsley sprigs, whole peppercorns, sugar, and 1 teaspoon salt with 2 cups cold water in medium saucepan. Place saucepan over medium heat and cook shrimp, stirring several times, until pink, firm to touch, and centers are no longer translucent, 8 to 10 minutes (water should be just bubbling around edge of pan and register 165 degrees on instant-read thermometer). Remove pan from heat, cover, and let shrimp sit in broth for 2 minutes.
Meanwhile, fill medium bowl with ice water. Drain shrimp into colander, discard lemon halves, herbs, and spices. Immediately transfer shrimp to ice water to stop cooking and chill thoroughly, about 3 minutes. Remove shrimp from ice water and pat dry with paper towels.
Whisk together mayonnaise, wasabi, celery, remaining tablespoon lemon juice, ginger, scallion, and sesame seeds in medium bowl. Cut shrimp in half lengthwise and then each half into thirds; add shrimp to mayonnaise mixture and toss to combine. Adjust seasoning with salt and pepper and serve.