Cook pasta according to package directions, except reduce cooking time by 2 minutes. Rinse pasta with cold water; drain again. Set aside.
In a large skillet heat 2 tablespoons butter and the oil over medium heat. Add leeks and 1/3 cup of the shallots; cook about 5 minutes or until tender. Stir in salt and thyme. Season to taste with pepper. Transfer to a small bowl. Stir in crabmeat; set aside.
Wipe out skillet with paper towels. For sauce, melt the remaining 6 tablespoons butter over medium heat. Add the remaining shallots; cook for 2 to 3 minutes or until tender. Stir in garlic; cook about 30 seconds more. Stir in flour; cook and stir for 1 minute. Whisk in milk until smooth. Cook and stir for 2 minutes more. Remove from heat. Add Parmesan cheese, if using, stirring just until combined. Stir in tomatoes, lemon juice, tarragon, and parsley. Stir 1 cup of the sauce into the crab mixture.
Preheat oven to 350 degrees F. Spoon about 1 tablespoon of the crab-sauce mixture into each pasta shell. Spread 1 cup of the remaining sauce onto the bottom of a 2-quart rectangular baking dish. Place filled pasta shells atop. Spoon the remaining sauce over pasta shells.
Bake, covered, about 30 minutes or until heated through. Serve immediately.