Bring 1/4 cup vinegar to boil with sugar and salt, stirring until sugar is dissolved and remove from heat.
In a dry, small skillet toast sesame seeds over moderate heat, stirring until golden and fragrant and transfer to small bowl (keep an eye that they don't burn).
Transfer rice to a large bowl and stir in vinegar mixture. Stir in sesame seeds, remainind 3 tbsp vinegar, oil, ginger, scallions, carrots and cucumber.
In a small bowl, stir wasabi into hot water and stir in cold water, soy and ginger juice. Serve salad sprinkled with remaining nori strips and drizzled with dressing.
In a separate bowl, mix spicy sauce with mayonnaise.
Bake rice crispies at 350F for about 5 minutes.
Salad may be prepared up to this point 1 day ahead and chilled and covered.
Bring salad to room temperature before proceeding. With scissors, cut nori into thin 2-inch long strips and add to salad.