Cactus, Chayote, and Green-Apple Salad

Put 1 cactus paddle, flat side down, on a work surface. Scrape horizontally from narrow base of paddle toward wider end with a vegetable peeler or knife to cut off thorns and brown spots. Dethorn paddle on other side in same manner, then trim all around edge. Repeat with remaining paddles, then cut paddles crosswise into 1/4-inch-thick slices.
Blanch cactus in a 6- to 8-quart pot of boiling salted water until just softened, about 1 minute. Immediately transfer with a slotted spoon to a bowl of ice and cold water to stop cooking. Drain in a colander and rinse under cold water. Pat dry, then transfer to a large bowl along with chayotes, apple, and scallions.
Cut peel and all white pith from 3 oranges with a sharp knife. Working over bowl containing cactus mixture, cut segments free from membranes, letting segments fall into bowl. Squeeze juice from membranes and remaining orange into bowl, then add lime juice, salt, and pepper and toss. Let stand until chayotes are wilted, about 30 minutes.
While salad stands, toast pumpkin seeds in 2 tablespoons oil in a 10-inch heavy skillet over moderately low heat, stirring frequently, until puffed and pale golden, 3 to 4 minutes. Remove from heat and stir in cayenne. Cool seeds in oil in skillet.
Drain salad in a colander set over a bowl, then transfer juice to a 1-quart saucepan and boil until syrupy and reduced to about 1/3 cup, 4 to 5 minutes. Remove from heat and whisk in remaining 3 tablespoons oil. Return salad to bowl and toss with dressing, pumpkin seeds with oil from skillet, and salt to taste.