Preheat the broiler. Tear 1 tortilla into pieces and transfer to a blender. Add the tomatoes, chicken broth, cilantro, chipotle and adobo, and cumin; puree until smooth.
Heat 1 tablespoon vegetable oil in a large skillet over medium-high heat. Add the pureed sauce and simmer, stirring occasionally, until slightly thickened, 6 to 8 minutes. Remove from the heat and season with 1/2 teaspoon salt and a few grinds of pepper.
Meanwhile, coat a 9-by-13-inch baking dish with vegetable oil. Mix the chicken, sour cream, 1/2 cup of each cheese and a pinch each of salt and pepper in a medium bowl. Warm the remaining 8 tortillas in the microwave as the label directs. Spoon about 1/4 cup of the chicken mixture down the center of each tortilla and roll up to enclose. Arrange side by side in the prepared baking dish and brush with the remaining 1 tablespoon vegetable oil.
Broil the enchiladas until lightly toasted, 3 to 5 minutes. Top with the tomato sauce and sprinkle with the remaining cheese. Broil until the sauce is bubbling and the cheese melts, 1 to 2 minutes. Top with avocado and scallions.