In a small bowl, stir together both chili powders and the salt. Dip the shrimp into the mixture, coating well on all sides. Place the coated shrimp in a shallow pan and pour the olive oil evenly over all. (Discard any chile coating that remains, as contamination could result for its later use.) Cover the shrimp and marinate in the refrigerator while you prepare the beans and tortillas.
Prepare fire in a charcoal grill. Preheat an oven to 250 degrees F.
To prepare the beans, combine the cumin and coriander seeds and the chili powder in a small, dry saute pan over medium heat. Roast until the aromas are released, about 2 minutes. Remove from the heat and let cool. Place the spices in a spice mill or coffee grinder until pulverized. Alternatively, pulverize in a mortar using a pestle.
Warm the olive oil in a large cast-iron skillet over low heat. Add the beans and ground roasted spices and cook, mashing the beans with the back of a wooden spoon and stirring frequently. Adjust to taste with salt. Continue to cook until the beans absorb the oil and flavors, about 10 minutes. Set aside, covered, in the oven until time to assemble the tostadas.
To ready the tortillas, heat the peanut oil in a skillet large enough to accommodate the tortillas laid flat, one at a time. When the oil is hot but not smoking, using tongs grasp a tortilla and slip it into the hot oil. Fry until crisp but not brown, about 2 minutes. It will curl up somewhat on the edges. Remove to paper towels to drain and reserve in the warm oven until time to assemble the tostadas. Repeat with the remaining tortillas.
To prepare the salsa, cut the tomatoes, tomatillos, avocado, onion and jalapeno into 1/4 inch dice. Place in a bowl and stir in the citrus juices and oil. Season to taste with salt.
Just before serving time, using tongs arrange the shrimp on the grill rack. Alternatively, skewer the shrimp on 2 parallel bamboo skewers for easy turning. Grill quickly until pink and cooked through, 2 to 3 minutes on each side.
To serve, place 2 fried tortillas on each individual plate. Place about 1/3 cup refried beans in the center of each tortilla. Arrange 3 grilled shrimp on top of each mound of beans. Spoon the salsa over and around the shrimp, using all of the salsa. Sprinkle the crumbled cheese over the top and garnish with cilantro or parsley sprigs. Serve immediately.