Prepare salsa: arrange oven racks in center and top half of oven, and preheat oven to broil. Trim about 1 inch from tip of corn to form a flat end. Place corn on baking sheet and transfer to top oven rack. Broil, turning once, until kernels are brown and blistered, about 18 minutes. Remove from oven and let cool to room temperature. Over a cutting board, stand corn upright on cut end and carefully slice kernels from cob in a downward motion. Transfer kernels to a medium bowl. Add cucumber, onion, tomatoes, peppers, zest and juice of 1 lime, oil, cilantro, 1/4 tsp cumin, and 1/4 tsp salt to bowl. Refrigerate until needed.
Preheat oven to 425 degees. Stack tortillas on top of each other and wrap in aluminum foil. Place packet on one side of a parchment lined baking sheet.
In a shallow bowl, add flour. In a separate shallow bowl, whisk egg whites, remaining cumin, and 1/4 tsp salt. In the bowl of a food processor, pulse rice cereal into a fine powder; transfer to a third shallow bowl. Mix brown rice cereal with panko crumbs. Pat halibut dry with a paper towel. Working with one piece at a time, coat each strip in rice flour, then egg mixture, and finally the rice cereal/panko mixture. Arrange strips in a single layer on empty side of baking sheet that you placed tortillas on. Generously mist tops of prepared fish with olive oil cooking spray and bake in center of oven for 6 minutes or until coating is golden and crispy.
Meanwhile, in a small bowl, combine yogurt, feta, and remaining lime zest and juice. Set aside. Chop up 1 avocado into small chunks and set aside as well.
To assemble tacos, carefully open foil packet of tortillas and allow steam to escape...they will be hot! Lay tortillas in a single layer on cutting board and mix avocado with salsa. Spoon avocado/salsa mixture onto center of each. Top each with 1 piece of halibut, yogurt-feta mixture, and finely shredded lettuce. Enjoy!