Bring a large pot of salted water to a boil add 1 tablespoon olive oil.
Heat the remaining tablespoon olive oil in a large skillet over medium-high heat. Add the garlic and onion and cook, stirring occasionally, until they start to soften, about 5 minutes. Add the crab and cook, stirring occasionally, until the crab starts to brown, 3 to 5 minutes. Scoop the contents of the skillet into a bowl and set aside.
Deglaze the skillet with the sherry and reduce for about 5 minutes. Add the butter to melt, then add the flour and cook, stirring, until there are no remaining clumps. Whisk in the heavy cream, milk, parsley, tarragon, thyme, mustard and paprika, followed by the Gruyere, Parmesan and 8 ounces of the Cheddar. Bring to a simmer, then lower the heat and simmer on low for 10 minutes. Season with salt and pepper.
Meanwhile, add the noodles to the boiling water and cook according to the package instructions. Reserve 1 cup pasta water, then drain the noodles and return them to the pot (I like to just leave a little pasta water in the pot when I drain it). Add the cheese sauce, cream cheese and crab and mix well. (If the mixture is too stiff, add some pasta water 1/4 cup at a time.) Season with more salt and pepper, if needed.
Pour the mixture into a 9-by-13-inch baking dish. Layer the remaining 4 ounces Cheddar evenly over the top. Bake until the cheese is nice and brown, 10 to 15 minutes. (Be careful, the cheese may bubble over in the oven.) Let rest for 15 to 20 minutes before serving. Sprinkle with parsley.
Seafood Mac and Cheese can be made ahead and held, then reheated in a 200 degree F oven for 15 to 20 minutes.