Make mayonnaise: Preheat oven to 350°F. Separate garlic cloves, place in a baking dish and drizzle with oil. Roast until softened, about 35 minutes. Cool in pan on a wire rack. Lower temperature to 200ºF. Peel garlic, add lemon juice and mash with a fork. Stir in mayonnaise.
Make crab cakes: Mix celery, scallions, crabmeat, mayonnaise and 2 cups bread crumbs; season with salt and pepper. Spread remaining bread crumbs on waxed paper. Gently form mixture into 8 1-inch-thick patties. Place each cake in bread crumbs and turn to coat.
Melt butter with oil in a large skillet over medium-high heat. Working in batches, cook crab cakes until golden brown, 2 to 3 minutes on each side. Keep warm in oven until ready to serve.