Special equipment: an 18-inch paella pan or large cast-iron skillet
In a small bowl, mix the paprika and garlic powder. Season the chicken, shrimp and calamari with the mixture. Brush both sides of the meat and seafood with 3 tablespoons of the canola oil. Let marinate 1 hour refrigerated.
Preheat a grill to medium-high heat.
Season both sides of the meat and seafood generously with salt and pepper. Grill the chicken a few minutes per side to crisp the skin, then move to a cooler part of the grill to finish cooking, about 10 minutes. Remove and place on a baking sheet. Grill the shrimp 1 to 2 minutes per side, then remove to the baking sheet. Grill the calamari a few minutes per side. Slice the calamari into rings on a cutting board and transfer to the baking sheet. Grill the lemon halves cut-side down until grill-marked, 3 to 4 minutes; transfer to the baking sheet. Set aside.
In a saucepan, combine the chicken stock and saffron threads. Bring to a simmer, whisking until the saffron dissolves. (This can be done on the grill as well, or on the stove inside and brought out to the grill area. The stock should be hot while you are making the paella.)
Place an 18-inch paella pan (or large cast-iron skillet) over the hottest part of the grill. Heat the remaining 2 tablespoons canola oil, add the onions and saute until translucent. Add the garlic and cook 1 to 2 minutes, stirring constantly. Add the chorizo and cook, stirring with a wooden spoon, until the fat starts to render and it begins to brown, 4 to 5 minutes. Stir in the rice, coating it in the chorizo fat and toasting it, about 2 minutes. Using a ladle, begin to add the stock, stirring constantly. Cook, continuing to add the stock as it absorbs, until the rice is al dente, 15 to 20 minutes. Once the stock is absorbed, stir in the peas. Now, stop stirring to allow the "soccarrat" to form (this is the crispy bottom on the rice; it's not burnt!). Allow the bottom to crisp up for about 5 minutes.
Arrange the chicken, seafood, lemon halves and peppers on top of the rice. Sprinkle the parsley and scallions over the top. Use a spoon or spatula to scrape up the crispy bottom of the rice and serve.