In a small bowl, combine the rice flour and 1/2 cup soda water. Stir until blended. Combine the flour, baking powder and cornstarch. Sift into a medium bowl. Stir in the remaining 2 1/2 cups soda water, salt and cayenne. Lastly add the rice flour mixture. Allow to rest in the refrigerator at least 1 hour before use. Check the consistency of the batter by dipping a piece of vegetable and cook in a preheated deep fryer at 350 to 375 degrees F. Additional soda water may be needed to adjust thickness.
In a bowl, dissolve the dry mustard and turmeric in the water and rice wine vinegar. Stir in the pickled ginger liquid, lemon juice, and sugar and whisk until well blended. Combine the chili, sesame and peanut oils. Slowly whisk in the oil to the mustard mixture until emulsified. Refrigerate until needed. This can be prepared up to 1 week ahead.
In a small bowl, whisk together the mirin, sugar, lemon juice, soy, and wasabi. Stir in the sesame oil and scallions. Use as needed.