Preparing the grill and the soup ingredients: About 30 minutes before cooking, prepare a charcoal fire, letting the coals burn until they are covered with a gray ash and are medium-hot. Bank the coals on 2 sides of the lower grate to prepare for the indirect cooking that follows.
In a 12 by 9-inch, heavy-duty aluminum-foil pan (or something similar), combine the potatoes, carrots, onion, garlic, garbanzo beans, and epazote. Position the pan in the center of the lower grate and surround with the coals. Pour water into the pan to about 1-inch from the top (it'll take about 5 cups). Position the cooking grate 8 inches above the coals and set an oven thermometer on it, if you have one.
Grilling the meat: Sprinkle the lamb liberally with salt. Lay the roast in the center of the cooking grate directly over the soup, cover the grill, and cook, maintaining a moderately low temperature (between 250 and 300 degrees F), checking every 30 minutes and adding coals as needed. The lamb will be beautifully smoky-roasted, it'll register about 170 degrees F on a meat thermometer and be fall-apart tender in about 2 1/4 hours. Be sure to check the slow-simmering soup that's capturing all those lamb juices periodically, to ensure the liquid level remains more or less the same, adding more water if it's needed.
Finishing the dish: With a couple of meat forks or spatulas, remove the roast to a platter. Sprinkle with salt and let rest, tented with foil, in a warm place for about 20 minutes.
Meanwhile, with the precision of a steady-handed circus performer, carefully remove the pan of soup from the bottom of the grill. Skim off the fat that is floating on the surface, then taste and season with salt, usually about 3/4 teaspoon. Stir in the cilantro and finely chopped chipotle and ladle into small, warm soup cups.
Scrape the salsa into a serving dish and sprinkle with cheese. Remove the string from the lamb. Slice the lamb into good, thick slabs and arrange on a warm platter that's lined or decorated with parsley, banana leaves, or lemon leaves. Strew the olives around the platter and carry to the table with a flourish. Serve each guest a cup of soup, and pass the meat, salsa, and lots of warm tortillas for everyone to make delicious soft tacos.