Make the pork rub: In a bowl, mix the garlic paste, ground cumin, black pepper, 2.5 tsp kosher salt (for pork rub), and 2 tbsp vegetable oil until a spreadable paste forms.
Prep the pork: Score the skin of the pork shoulder in a crosshatch without cutting into the meat. Rub the paste all over, working it into the scored skin and the meat. Place uncovered on a rack set over a pan and refrigerate 8–12 hours.
Roast low and slow: Heat the oven to 325°F (165°C). Set the pork shoulder on a rack in a roasting pan, pour 1 cup water into the pan, and roast until the meat is very tender and a skewer slides in with little resistance, 2.5–3 hours (internal temp around 175–185°F).
Make the escabeche while the pork roasts: In a large bowl, combine the sliced red onions, carrots, and rocoto pepper. Add the white vinegar, 0.5 cup water, 1 tsp kosher salt (for escabeche), and 1 tbsp vegetable oil. Toss and lightly massage the onions until they wilt slightly, then let stand 30–60 minutes, tossing occasionally.
Crisp the skin: Increase oven temperature to 450°F (230°C). Return the pork to the oven and roast until the skin is blistered, crackly, and deep golden, 20–30 minutes. Rest the pork 20 minutes.
Slice and prep bread: Thinly slice the pork, chopping the crackling into bite-size pieces. Split the marraqueta rolls (warm briefly if desired).
Assemble and serve: Pile sliced pork onto the rolls, add pieces of crackling, and top generously with the onion–carrot–rocoto escabeche. Press to close and serve immediately with llajua on the side.