Preheat the broiler. Heat 1 tablespoon vegetable oil in a large ovenproof skillet over medium-high heat. Add the collards and cumin and cook until heated through, 2 to 3 minutes. Remove to a bowl (reserve the skillet). Add the chicken, half the scallions and 3/4 cup cheese to the bowl. Season with salt and pepper and toss.
Wipe out the reserved skillet and add the remaining 1 tablespoon vegetable oil. Stir in the salsa, heavy cream and 1/2 cup water; reduce the heat to medium and bring to a simmer. Dip the tortillas briefly in the sauce and lay on a baking sheet. Divide the chicken mixture among the tortillas and roll up.
Remove 1/2 cup sauce from the skillet; arrange the enchiladas seam-side down in the skillet. Spoon the reserved sauce on top. Broil until the enchiladas are just crisp and lightly browning, about 2 minutes. Sprinkle with the remaining 1 cup cheese and broil until bubbling, about 1 minute. Top with the remaining scallions, the avocado, cilantro and sour cream.