Preheat a grill to medium heat.
Bring a large pot of water to a boil and generously season with salt. Cook the pasta 1 to 2 minutes less than the package directions. Reserve 1/4 cup of the pasta water, then drain and set aside.
Meanwhile, use a paper towel to pat dry the shrimp and scallops. Season with salt and pepper on both sides and brush with the olive oil. Grill just until grill marks appear, about 1 minute per side (they will be undercooked); reserve. Grill the lemon halves until softened and grill marked, 5 to 8 minutes; reserve.
Place a large saucepan on the grill (this can also be done inside on the stove). Heat the marinara sauce until hot, then add the capers and lemon zest and juice. Add the clams and cover. Cook until the clams begin to open, about 5 minutes. Add the pasta and reserved pasta water to the sauce and toss to coat. Add the parsley and reserved shrimp and scallops and continue to toss until the pasta is evenly coated, the shrimp and scallops are cooked through and all the clams are open, about 1 minute. Serve with the grilled lemon halves.