Put beans, reserved bean liquid, 1/2 cup of scallions, chili powder, garlic, and cumin in a medium bowl. Mash with potato masher to make a chunky puree. To assemble; cut corn tortillas in half, line inside of baking dish with four tortilla halves. Spread bean mixture on tortillas, top with four tortillas halves, spoon corn on top, and sprinkle with half of the cheese and all the chilies. Top with remaining tortilla halves, spread with remaining bean mixture, spoon tomatoes and liquid over beans, and sprinkle with remaining cheese. Cover tightly with foil. Bake 40 to 45 minutes at 350 degrees. To serve, top with dollops of sour cream and sprinkle with remaining 1/4 cup scallions.