1. With a sharp knife, cut off and discard stem end of each ear of corn down to kernels. Carefully pull off husks without tearing; rinse and cover with damp towels to keep from drying out. Pull off and discard silks from corn; rinse ears. Holding each ear upright, cut off kernels; you need 7 cups (save any extra for other uses). Discard cobs.
2. Working in batches in a blender or food processor (fill blender no more than half full), whirl corn kernels, dehydrated masa flour, and olive oil until corn is finely ground. Scrape mixture into a bowl. Stir in cheese, chilies, and 1 1/2 teaspoons salt.
3. Working with a few at a time, overlap corn husks, concave side down, to line a shallow 3- to 3 1/2-quart casserole, letting ends stick up 3 to 4 inches above rim; drop spoonfuls of the corn mixture onto husks as you go to hold them in place. Discard extra husks. Spoon remaining corn mixture into casserole and spread level. Fold ends of husks over corn mixture. Cover casserole tightly with foil.
4. Bake in a 350° regular or convection oven until corn mixture is steaming and slightly firm to touch in the center (uncover to check), 55 to 60 minutes. Let casserole stand, covered, for 10 minutes.
5. Meanwhile, cut chorizo into 1/4-inch-thick slices. In a 10- to 12-inch frying pan over medium-high heat, stir chorizo often until browned, about 5 minutes. Spoon off any fat and discard. Add cactus to pan and stir until hot, about 1 minute.
6. Uncover casserole and unfold husks. Spoon sausage mixture over center of tamale pie and sprinkle cilantro on top. Serve with green salsa. Add salt to taste.