Melt 125g butter and 1/2 cup olive oil together in a saucepan over medium heat. Stir in the garlic and cook until fragrant, about 3 minutes. Stir in the 1 1/4 cups of parsley and cook for 1 minute more. Season with lemon juice, chilli flakes, salt and pepper; set aside.
Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Meanwhile, bring a few cups of water to a boil in a saucepan over high heat. Stir in the oysters and turn off the heat. Allow the oysters to stand for 3 minutes, then drain and set aside. Melt 65g butter and 1 tablespoon olive oil together in a large frypan over medium-high heat. Stir in the prawns and cook until they turn pink and begin to firm. Add the crab meat and drained oysters; continue cooking until the prawns have turned opaque in the centre and the crab is hot.
Toss the seafood with the drained pasta and season to taste with salt and pepper. Pour in the sauce and toss gently to coat. Sprinkle with remaining 1/4 cup of chopped parsley before serving.