Make the dipping sauces: Whisk the mayonnaise, curry paste, lime zest and 1 tablespoon of the lime juice in a small bowl until combined. Cover and refrigerate until ready to use. Combine the remaining lime juice with the garlic, fish sauce, sugar, Sriracha and 1 tablespoon cold water in a separate small bowl, stirring to dissolve the sugar. Stir in the cilantro, cover and refrigerate until ready to use.
Prepare the crab fries: Heat 2 inches of vegetable oil in a large pot or Dutch oven over medium heat until a deep-fry thermometer registers 360 degrees F. Set a wire rack on a baking sheet.
Meanwhile, cut the crabs into quarters with kitchen shears; transfer to a paper towel-lined plate. Whisk the cake flour with 1/2 teaspoon salt in a medium bowl; add the seltzer and scallions and whisk just enough to make a lumpy batter.
Working in batches, dip the crab pieces in the batter, then carefully add them to the hot oil and fry until golden and crisp, about 2 minutes. (Use a mesh screen if you have one; the crabs may splatter.) Remove the crab fries with a strainer or slotted spoon and transfer to the wire rack; season with salt. Let the oil return to 360 degrees F between batches. Serve the crab fries with the sweet chili sauce, curry mayonnaise and cilantro-lime sauce.