For the bechamel sauce: Melt the butter in a saucepan. Whisk in the flour and cook for 2 to 3 minutes, stirring constantly. Slowly whisk in the milk. Simmer for 20 minutes, stirring frequently to avoid burning. Season with salt and white pepper, then strain through a chinois and hold for later use.
For the seafood alfredo pasta: Bring a large pot of water to a boil to reheat the cooked pasta. In a saute pan or small saucepan, heat the bechamel sauce to a simmer. Add the Parmesan and stir to melt the cheese. Add the crabmeat and toss to warm.
Sprinkle the shrimp with salt and pepper. Heat a large saute pan over high heat. Add the grapeseed oil and then the shrimp to the pan. Cook for 1 to 2 minutes, then flip the shrimp and finish cooking, another 1 to 2 minutes. Remove the shrimp, leaving the oil in the pan. Hold the shrimp for plating.
Add the sliced garlic to the shrimp pan and saute until it just begins to brown around the edges. While the garlic is cooking reheat the pasta in the boiling water, about 1 minute. Strain the pasta, add it to the pan with the garlic and toss.
Add the bechamel and crab to the pasta and toss. Add the lemon juice, Chicken stock and cream and sprinkle with salt and pepper. Add the egg yolks and toss to incorporate. Cook for 1 minute to help thicken the sauce. Add the shrimp and toss.
Divide the pasta and shrimp evenly between serving plates and garnish with chopped chives.