Make the crab cakes: Place half of the cracker crumbs in a large bowl. Drizzle in the egg, then stir in the mayonnaise, Worcestershire sauce, lemon juice, parsley, celery leaves, celery seeds, mustard powder, chili powder, 1/2 teaspoon salt, and pepper to taste. Gently fold in the crabmeat. Form into six 3-inch-wide patties (about 1/2 cup crab mixture each). Spread the remaining cracker crumbs on a plate; press the crab cakes in the crumbs on both sides and around the edges. Transfer to a baking sheet. Cover and refrigerate until firm, 2 to 6 hours.
Make the sauce: Mix the mayonnaise, lemon juice, pickles, Worcestershire sauce, hot sauce and scallions in a small bowl. Season with salt and pepper. Cover and refrigerate until ready to serve.
Position a rack in the upper third of the oven and preheat to 450 degrees F. Brush the crab cakes with the 6 tablespoons melted butter and bake until golden, 15 to 20 minutes.
Meanwhile, make the salad: Gently toss the lettuce, endive, tomatoes, celery, scallions, chervil and parsley in a large bowl; drizzle with the olive oil, season with salt and pepper and toss again. Serve with the crab cakes and sauce.