Heat a large cast-iron skillet over medium heat. Add garlic to pan; cook 8 minutes or until skins are well charred, turning occasionally. Remove cloves from pan; cool 5 minutes, and discard skins. Combine garlic, rind, juice, achiote paste, and 2 teaspoons oil in a mini chopper; process until smooth.
Sprinkle 1 teaspoon salt over pork. Brush achiote mixture evenly over pork. Place pork in a medium Dutch oven or ovenproof saucepan; cover and bake at 250° for 6 hours. Remove from oven, and let stand for 30 minutes.
While pork cooks, place onion in a medium bowl. Combine 1/2 cup water, vinegar, agave, and remaining 3/4 teaspoon salt in a saucepan over medium-high heat; bring to a boil, stirring until salt dissolves. Pour vinegar mixture over onion; stir. Cover bowl with plastic wrap; cool to room temperature, rearranging as needed to keep onion submerged. Chill at least 1 hour.
Place pork on a cutting board; shred with 2 forks. Discard fat.
Heat a large cast-iron skillet over medium-high heat. Add 2 tablespoons oil to pan; swirl to coat. Add 1 cup shredded pork to pan in an even layer; cook 3 minutes or until a crust develops, turning once. Place crisped pork on a platter. Repeat procedure with remaining shredded pork.
Place 2 warm tortillas on each of 6 plates. Top each tortilla with 1 1/2 ounces crisped pork, 1 tablespoon Tomatillo Salsa, and 1 tablespoon pickled red onion.