In a 1-gallon zip-top bag, combine chipotle through cumin; squish bag until mixed. Add pork and shake/squish until it’s evenly coated. Seal bag and refrigerate for at least 2 hours or up to 1 day. Remove pork from fridge about 30 minutes before braising and preheat the oven to 325°F.
Place marinated pork in a small (2 – 3-quart) Dutch oven. Sprinkle smashed garlic over pork and pour water down the sides of the pan. Cover pan with lid and place in the preheated oven. Braise for 3 hours or until very tender.
Discard garlic and remove pork pieces to a large, heatproof dish to cool. Strain braising liquid through a mesh sieve, separate and discard fat, and reserve 1 cup of the liquid (if needed, add water to make 1 cup). Once pork is cooled to room temp, shred with hands or two forks; cover and refrigerate until ready to use.
DO AHEAD: Meat can be braised several days in advance — I highly recommend doing so!!
3 cups water Guajilloed braising liquid from pork (more info in instructions below)
14 1⁄2 ounce can diced tomatoes
1⁄2 teaspoon ground cumin
1 1⁄2 teaspoons Kosher salt
MAKE SALSA ROJA:
In a medium pan (4-quart) combine chiles, water, reserved braising liquid, tomatoes, cumin and salt. Set over high heat and bring to a boil. Reduce to low and simmer for about 30 minutes or until the ingredients are soft.
Transfer the mixture to a blender or food processor (make sure to cool it off before blending). I have a hand mixture blender and that allows me to blended it while hot without any issues. Blend until smooth; set aside.
Mix shredded pork with 2 cups (454 grams or 16 ounces) of the sauce. Set remaining sauce aside to serve with tamales.
DO AHEAD: Sauce can be made several days in advance, but only after the pork is braised (you need the reserved cooking liquid). Toss with pork as instructed, and store remaining sauce in a separate container.