Preheat oven to 375°.
To prepare sauce, discard husks and stems from tomatillos. Bring 2 quarts water to a boil in a large Dutch oven. Add tomatillos; cook 2 minutes. Drain. Place tomatillos, avocado, and next 5 ingredients (avocado through garlic) in a blender; process until smooth. Strain mixture through a sieve into a medium bowl; discard solids.
To prepare enchiladas, combine the cheese, 1/4 teaspoon salt, and crab. Warm tortillas according to package directions. Spoon about 1/2 cup crab mixture down center of each tortilla; roll up. Place, seam-side down, in an 11 x 7-inch baking dish coated with cooking spray. Cover and bake at 375° for 12 minutes or until thoroughly heated.
To prepare topping, combine lettuce and remaining ingredients. Serve enchiladas with sauce and topping.