Measure 14 quarts of water into a huge (30-quart) pot and add the corn (either the rinsed nixtamal or the dried corn) and garlic. Bring to a boil, partially cover the pot and simmer gently over medium-low heat until the corn is thoroughly tender - at a minimum allow about 3 hours for nixtamal, about 5 hours for dried corn. Add water as necessary to keep the water level more or less constant. Slower, longer cooking only means better pozole, as evidenced by the fact that in many places in Mexico huge pots of the fragrant mixture simmer for wood fires overnight before a fiesta.
While the corn is simmering, cook the meat. Place all the meats in another large pot, cover with 8 quarts of water, add 1/4 cup salt and bring to a boil, skim off the grayish foam that rises during the next few minutes, then add half of the chopped onions. (Scoop the remaining onion into a strainer, rinse under cold water, shake off the excess, then place in a serving bowl, cover and refrigerate for serving.) Partially cover the pot and simmer over medium-low heat until all the meat is thoroughly tender, about 3 hours. Remove the meat from the broth and let cool. Or, if time allows, cool the meat in the broth for the best flavor and texture, then remove it.
Skim the fat from the broth; you'll have about 4 quarts broth. Add it to the corn, along with 2 tablespoons salt, partially cover and continue simmering until the corn is completely tender and you're ready to serve.
Pull off the meat from the pork shanks and pull the shoulder meat into large shreds. Collect in a large roasting pan. Cut the bones and knuckles out of the trotters. Discard the bones and knuckles, then chop what remains into 1/2-inch pieces. Add the shredded meat (there will be about 12 cups meat in all). Cover and refrigerate if not serving within an hour.
When you're ready to serve, heat the meat in a 350 degree oven until just warm through (no need for the meat to get really hot), about 20 minutes. Taste the pozole and season with additional salt if you think necessary. Set out bowls of the condiments for your guests to add to their steaming, fragrant bowlfuls al gusto: the reserved onion, the ground chile, lime wedges, sliced cabbage or lettuce, sliced radishes and oregano.
Ladle portions of the pozole into large soup bowls, letting your guests add the condiments to suit their own tastes. Before sprinkling it over the bowl, each guest should powder the whole-leaf oregano by rubbing it between his or her palms.