BRING A LARGE pot of salted water to a boil for the pasta. Preheat the oven to 400°F. Butter a 9- by 13-inch baking dish. When the water is boiling, add the elbow macaroni and cook until very al dente, several minutes shy of the package cooking time. Drain, rinse, and pat dry and put in a large bowl.
In a large saucepan over medium heat, melt 4 tablespoons of the butter. Add the mushrooms and season with salt and pepper. Cook until the mushrooms have given up their liquid. Increase the heat to boil away any excess liquid, about 5 minutes. Add the scallions, thyme, and garlic and cook until the scallions are wilted, about 3 minutes.
Sprinkle the vegetables with the flour and cook, stirring, until lightly browned, about 1 minute. Add the wine and cook until just absorbed. Pour in the chicken broth and half-and-half. Bring to a simmer and season with salt, pepper, and the nutmeg. Simmer until the sauce is thickened and has lost the raw flour taste, about 7 minutes.
Stir in the chicken, peas and carrots, and parsley and simmer just to heat the chicken through. Off the heat, whisk in the cream cheese until smooth. Pour the chicken and sauce into the bowl with the macaroni and toss well. Spread into the prepared baking dish. In a small bowl, toss together the breadcrumbs and Parmesan. Sprinkle the breadcrumbs over the top of the baking dish. Dot the top with the remaining 2 tablespoons butter, cut into small pieces. Bake until the casserole is very bubbly and the top is crispy and golden brown, about 25 minutes.