Pierce the potatoes a few times and rub them with oil. Sprinkle with salt. Place on a baking sheet and baked in a 400 degree oven for 55 to 60 minutes. Let cool until cool enough to handle.
Place cream cheese in a large bowl.
Cut a sliver of skin from the top of each potato and discard it. Scoop the insides of the potatoes out and place in the bowl with the cream cheese.
Add 1/2 teaspoon of salt and 1/2 teaspoon of pepper and use a potato masher or hand-held electric mixer to get the mixture smooth. Return mixture to potato skins. Cover with foil and place in a 200 degree oven until the sauce is ready.
Season shrimp with 1 teaspoon of Cajun seasoning and 1 teaspoon of garlic powder.
To make sauce, melt 2 tablespoons of butter in a large skillet. Add garlic and cook for 30 seconds.
Add shrimp and cook until pink. Remove shrimp from skillet and set aside.
Add remaining butter to skillet. When melted, add the flour and cook and stir for 1 minute.
Gradually whisk in half-and-half and bring to a simmer.Add Cajun seasoning, seafood seasoning, onion powder, 1/2 teaspoon of pepper and 1/4 teaspoon salt. Simmer for a few minutes.
Stir in velveeta cheese, mozzarella cheese, Monterey Jack and Parmesan. Turn heat to low and stir until cheese is melted.
Add the shrimp and crab and stir to mix them in. When heated through, remove from heat.
Spoon sauce over potatoes and sprinkle with green onions.
Notes
To save time, make the sauce while the potatoes are baking.