For the seafood: Place the octopus in a pressure cooker and cover with water by 4 inches. Add the salt, 1/4 cup olive oil, 3 sprigs thyme and 1 clove garlic. Lock the lid according to the manufacturer's instructions and pressure cook for 40 minutes. Follow the manufacturer's instructions for releasing the pressure. Drain, let cool and slice 1-inch thick.
Meanwhile, place the remaining 3 cups extra-virgin olive oil in 8-quart saucepot with the lemon peels, remaining 3 sprigs thyme and 1 garlic clove. Heat to 190 degrees F. Poach the shrimp in the oil until cooked through, about 3 minutes, then remove with a slotted spoon to a bowl. Poach the calamari in the oil until cooked through, about 3 minutes. Remove with a slotted spoon to another bowl and allow the shrimp and calamari to cool in the fridge.
For the pickled chiles: Bring the vinegar to a boil in a saucepot with the peppercorns and bay leaf. Turn off the heat and let cool. Pour over the chiles in a bowl and allow to pickle for 20 minutes.
For the tostadas: Clip a deep-frying thermometer to a large, heavy-bottomed skillet. Heat the vegetable oil to 375 degrees F. Fry tostadas until golden, 1 to 2 minutes each. Remove with a slotted spoon and drain on paper towels to cool.
For plating: Add all the chilled seafood, chiles, cucumber, radish, lime juice and chopped cilantro in a large bowl and mix together. Place a large scoop of seafood on a tostada, then garnish with cilantro leaves and drizzle with the oil. Repeat with remaining ingredients to make 7 more tostadas.