For the pasta and filling: Bring a large pot of water to a boil; add a generous amount of salt. Cook the pasta shells until al dente according to the package instructions. Drain and set aside.
Meanwhile, add the oil and butter to a large skillet and heat over medium heat until the butter is melted. Add the red pepper flakes, garlic and onions and cook until the onions are translucent, 2 to 3 minutes. Add the shrimp and cook, stirring, for 1 to 2 minutes. Add the wine, lemon juice and 1/2 teaspoon each salt and pepper. Stir and simmer until the shrimp is cooked through, about 1 minute. Set aside on a plate to cool completely.
Add the cream cheese, ricotta, Parmesan, parsley, egg and 1/2 teaspoon each salt and pepper to a large bowl. Using a spatula, mix together until well combined. Fold in the shrimp and set aside while you make the sauce.
Preheat the oven to 375 degrees F.
For the sauce: Using the same skillet over medium heat, melt the butter and sprinkle over the flour. Cook, whisking, until the roux turns golden brown, 3 to 4 minutes. Pour in the milk and cream, whisking constantly, and cook until thickened, a few minutes. Add the garlic and 1/4 teaspoon each salt and pepper. Add the parsley and set aside.
To assemble: Add 1 cup of sauce to the bottom of a 9-by-13-inch baking dish. Using a spoon, fill each of the jumbo shells generously with the shrimp/cream cheese mixture. Lay 20 of the stuffed shells face down in the baking dish. Top with an additional 2 cups of sauce. Sprinkle over half of the shredded mozzarella. Bake, uncovered, until bubbly and hot, about 25 minutes.
For the second casserole: Assemble a second 9-by-13-inch baking dish with the remaining ingredients as above, then cover with plastic wrap and aluminum foil and freeze. When ready to bake, preheat the oven to 375 degrees F. Bake, uncovered, until bubbly and cooked through, 35 to 40 minutes.