Fricassee of Game Hen with Creamy Leeks and Vadoum

Heat oil in a heavy medium pot over medium-high heat until it shimmers. Cook backbones and wing tips until well browned on all sides. Add water and bring to a boil, skimming froth occasionally. Add remaining ingredients and boil, uncovered, 30 minutes. Strain through a fine-mesh sieve into a clean saucepan, pressing on and then discarding solids. Boil stock until reduced to about 2 cups.
Pat hens dry and season with 1 teaspoon salt and 1/2 teaspoon pepper. Heat oil in a deep 12-inch heavy skillet over medium-high heat until it shimmers. Brown hens in batches (without crowding), turning once, about 8 minutes per batch. Transfer to a plate.
Halve white and pale green parts of leeks lengthwise, then cut into 1-inch pieces and wash. Pour off all but 1 tablespoon fat from skillet, then cook leeks with 1 1/2 teaspoons salt and 1/4 teaspoon pepper over medium-high heat, stirring occasionally and scraping up brown bits, until leeks begin to soften, about 5 minutes. Add wine and bring to a boil. Stir in stock, cream, and vadouvan. Nestle hens, skin sides up, into leeks. Cover with a round of parchment paper. Gently simmer 10 minutes. Remove parchment and simmer until hens are cooked through, about 10 minutes more. Transfer hens to a platter and simmer sauce to thicken slightly if desired.