In a bowl combine shrimp, 2 tablespoons of the lime juice, 2 teaspoons of the fish sauce, 1 tablespoon of the green chilies, and 1 teaspoon of the finely-minced lemongrass. Mix well and let marinate while you prepare the soup.
In a large saucepan heat oil over high heat. Add shrimp shells and cook, stirring occasionally, 5 minutes, or until shells have begun to brown. Add chicken stock, 2 tablespoons of the green chilies, the 2 tablespoons sliced lemongrass, kefir lime leaves, coriander roots and stems, galanga root, shallots and tamarind pulp; bring to a boil. Reduce heat to simmering, cover and cook 30 minutes.
Into a clean saucepan strain soup through a fine sieve, pressing on solids to extract as much liquid as possible; discard solids. Heat over medium-high heat and add remaining 2 tablespoons lime juice. remaining 2 teaspoons fish sauce, remaining 1 tablespoon green chilies, remaining teaspoon minced lemongrass, straw mushrooms, coriander leaves, and scallions; stir well. To serve, add shrimp in their marinade and cook 30 seconds or until shrimp turn pink and are cooked through.