Prepare the biscuit dough by mixing flour, baking powder, baking soda, and salt. Cut in cold butter until crumbly, then stir in buttermilk just until combined. Add cheddar if desired. Pat the dough to ¾-inch thickness, cut into 8 rounds, and bake at 425°F (220°C) for 12–14 minutes until golden.
For the seafood nacho filling, sauté onion, garlic, and bell pepper in olive oil and butter until fragrant. Add shrimp and cook until pink, then stir in crab.
Season with smoked paprika, chili powder, cayenne, salt, and pepper. Sprinkle flour, pour in milk, and whisk until thick. Stir in cream cheese and Monterey Jack until smooth, then finish with lime juice and cilantro.
Slice biscuits in half, spoon seafood filling over the bottoms, and top with cheese. Broil until bubbly, then garnish with sour cream, jalapeños, crushed tortilla chips, and green onions. Serve open-faced or sandwich-style.