Rip one of the corn tortillas into pieces and place in a blender with the crushed fire-roasted tomatoes, chicken broth, chipotle peppers, 2 tablespoons cilantro, cumin, 1/2 teaspoon salt, and 1/4 teaspoon black pepper; blend until completely smooth to make the sauce.
Heat 1 tablespoon of vegetable oil in a saucepan over medium heat, add the blended sauce, and simmer gently for about 8 minutes until slightly thickened; then remove from heat and set aside.
Preheat the oven broiler. In a medium bowl, combine the shredded rotisserie chicken, 2/3 cup sour cream, 1/4 cup sliced green onion, and 1 cup of the shredded Mexican cheese; season with salt and pepper to taste.
Place the remaining 8 tortillas on a microwave-safe plate, cover with a damp paper towel, and microwave for 30 seconds to soften them.
Grease a 9x13-inch baking dish. Spoon about 1/3 cup of the chicken mixture down the center of each softened tortilla, roll up, and place seam-side down in the prepared dish.
Brush the tops of the rolled enchiladas with the remaining 1 tablespoon vegetable oil, then broil 3–5 minutes just until the tortillas begin to toast.
Pour the reserved sauce evenly over the enchiladas, sprinkle with the remaining 1 cup shredded cheese, and return to the broiler until the cheese is completely melted.
Garnish with remaining cilantro and sliced green onion, drizzle with additional sour cream if desired, and serve with sliced avocado.