Make the fish stock. Heat the oil in a large stockpot or Dutch oven over medium heat and add the carrots, celery, leeks, and onions. Cook, stirring, until almost tender, about 5 minutes. Stir in the garlic and cook for 2 to 3 minutes, or until fragrant but not brown. Stir in the tomatoes and cook, stirring, for about 10 minutes. Add the whole rockfish, halibut bones, shrimp, crayfish, and parsley. Add enough water to cover by 2 inches and bring to a boil. Skim off any foam, then reduce the heat and simmer for 20 to 30 minutes, or until the meat is separating from the bones. Strain. Return the stock to the pot, bring to the boil, and reduce to 2 ½ cups, 45 to 60 minutes. Reduce the heat to medium low; adjust seasoning with the salt.
Peel the shrimp, leaving the shells on the tails. Remove the heads, setting half aside (about 32). Add the remaining heads to the reduced stock and simmer for 5 minutes. Strain; keep warm until ready to serve. Cook one of the potatoes in a medium pot in 1 quart of boiling water with 2 teaspoons of salt until tender, about 30 minutes. Drain and return to the pot. Cover and let steam for 5 minutes, then mash, using some of the fish broth to moisten. Season with ½ teaspoon of salt and the pepper. Keep warm.
Make the motar mix. Place the almonds, garlic, parsley, saffron threads, and salt in a mortar and mash to a paste. Add the fired bread and ñora peppers and continue to mash. Drizzle in the oil, stirring, and add the powdered saffron. Set aside and allow the flavors to meld.
Cook the remaining half of the heads quickly with ½ teaspoon of salt and 1 tablespoon of oil on a griddle or in a hot skillet over medium-high heat until they are crisp and red, 3 to 4 minutes. Keep warm until ready to serve.
Heat the oil in a Dutch oven or deep skillet until 375°F. Meanwhile, slice the remaining potatoes into ¼ -inch slices. Heat 2 tablespoons of oil in two large sauté pans over medium-high heat. Fry the potato slices in two batches in the Dutch oven, until golden brown and crisp. Season with ½ teaspoon of salt and drain on paper towels. While the second batch of potatoes is frying, cook the shrimp in the sauté pans just until pink, about 4 minutes.
Place about 2 tablespoons of mashed potatoes on each serving plate. Top with 3 shrimp heads and 6 tails per person. Place a crisp potato slice between each shrimp. Drizzle a line of the mortar mix over the shrimp (about 2 teaspoons per portion). Heat briefly under a broiler until the mortar mix is golden brown. Spoon a small ladle (1/4 cup) of stock over the top and sprinkle with sea salt.