Assemble ingredients, and prepare filling. Place pork, water, 2 onion wedges, 3 garlic cloves, and 1 teaspoon salt in a Dutch oven; bring to a boil. Cover, reduce heat, and simmer 1 hour, or until meat is fork-tender. Drain in a colander over a bowl, reserving cooking liquid. Remove meat, discarding remaining solids; cool. Shred pork with 2 forks. Sprinkle with 1/2 teaspoon salt, cumin, and pepper (adjust seasonings, if necessary).
To make salsa, combine remaining onion wedge, cilantro, 1/2 cup broth, tomatillos, 3 garlic cloves, and jalapeño in a saucepan; bring to a boil. Reduce heat; simmer 8 to 10 minutes, or just until vegetables are tender. Let stand for 5 minutes.
Transfer salsa to a blender; add a pinch of salt, and pulse until thick.
Meanwhile, place corn husks in a large bowl of hot water, and weigh down with another bowl. Soak at least 30 minutes. Drain husks; rinse with cold water. Drain and pat dry. Tear smaller husks lengthwise into 40 (1/2-inch-wide) strips.
Prepare masa by combining masa harina, broth, water, and 1/2 teaspoon salt in a medium bowl. Stir in reserved cooking liquid and oil (dough should be thick and sticky).
Open 1 large husk, curved side up. Place 1/3 cup masa mixture on center of husk; spread evenly with fingers into a rectangle of about 4 x 6 inches. Top masa with about 2 tablespoons shredded pork. Spoon 2 tablespoons salsa over pork. Fold long sides of husk over filling. Fold ends of husk over, overlapping to enclose tamale. Tie a husk strip around each end of packet. Repeat with remaining husks, masa, pork, and salsa (you may have extra husks and masa).
Place vegetable steamers in 2 large skillets or woks. Arrange 10 tamales in each steamer, or stack in a double-layered bamboo steamer in 1 skillet. Add water to skillets to a depth of 1 inch, and bring to a boil. Cover and steam 30 minutes, checking water level occasionally. Remove tamales from steamers, and let stand 5 minutes. Serve any remaining salsa with tamales.