Preheat oven to 375°F. Lightly coat a 13- x 9-inch baking dish with cooking spray; set aside. Melt butter in a large skillet over medium. Add collards, onion, and minced garlic; cook, stirring often, until collards are cooked to desired degree of tenderness, 10 to 15 minutes. Stir in cream cheese, milk, sugar, salt, pepper, and 1 cup of the cheese; cook, stirring constantly, until heated through, about 4 minutes. Remove from heat, and stir in chicken.
Whisk together enchilada sauce and sour cream in a small bowl. Spoon 1 cup sauce mixture into prepared baking dish.
Wrap tortillas in damp paper towels, and tightly wrap with plastic wrap. Microwave at HIGH until tortillas are softened, about 1 minute. (Softening the tortillas prevents them from tearing.)
Spoon about 1/3 cup collard green mixture just below the center of each tortilla. Fold opposite sides of tortilla over filling, and roll up. Place enchiladas, seam side down, on sauce mixture in baking dish. Top with remaining sauce mixture and remaining 1 cup cheese.
Bake in preheated oven until light brown and bubbly, 25 to 30 minutes. Garnish with sliced radishes and jalapeños and cilantro leaves.