Heat oil in a large saucepan over medium heat. Add scallions, lemongrass, and chile and cook until softened, 3-4 minutes. Add tomatoes and cook until softened, 5-6 minutes. Stir in chicken stock, coconut milk, orange juice, and fish sauce. Bring to a boil, reduce heat, and simmer, stirring occasionally, until flavors meld, about 15 minutes. Let cool slightly. Working in batches, puree soup in a blender until smooth, about 1 minute per batch. Stir in 1 tablespoon lime juice. Season to taste with salt and more lime juice, if desired. Chill, if desired. Or ladle hot soup into wide shallow bowls. Garnish with crabmeat, pea tendrils, and snow peas.