Make the Southwestern sauce: In a small bowl, whisk together mayonnaise, sour cream, 2 tbsp finely chopped red bell pepper, 1 tsp cayenne (ground red pepper), 3/4 tsp garlic powder, 3/4 tsp paprika, 2 tsp rice vinegar, and 1/4 tsp chili powder. Cover and refrigerate.
Make the black bean-corn salsa: In a medium bowl combine thawed corn, drained black beans, 1/4 cup finely chopped green bell pepper, 1/4 cup chopped red bell pepper, 1/4 cup finely chopped red onion, 1/4 cup chopped cilantro, 2 tbsp rice vinegar, 2 tbsp finely chopped jalapeno, 1 tbsp lime juice, 2 tbsp olive oil, 3/4 tsp salt, and 3/4 tsp black pepper. Stir to combine and refrigerate until ready to use.
Preheat a grill or grill pan to medium-high. Brush each chicken breast with 1 tbsp olive oil and season both sides with 1/4 tsp ground cumin, 1/4 tsp garlic powder, 1/4 tsp salt, and 1/4 tsp black pepper.
Grill the chicken 4 to 5 minutes per side, or until cooked through and the internal temperature reaches 165°F (74°C). Remove from grill.
Preheat the oven to 350°F (175°C). Place the grilled chicken breasts in a lightly greased baking dish.
Top each breast with about 1/3 cup Southwestern sauce, then about 1/2 cup of the black bean-corn salsa, and finally divide the shredded cheeses evenly (about 1/4 of the total cheese mixture per breast).
Bake for 7 to 8 minutes, or until the cheese is melted and bubbly.
Serve each chicken breast topped with tortilla strips and diced avocado.