Make the pernil:
Process garlic, oregano, vinegar, salt, sazón, and adobo seasoning in a food processor until mixture forms a paste, about 45 seconds, stopping occasionally to scrape down sides of bowl. Alternatively, crush mixture using a mortar and pestle to form a paste.
Using a sharp knife, score pork skin and fat cap about 1/4 to 1/2 inch deep in a diamond pattern, spacing cuts 1 1/4 inches apart. Cut 8 (2 1/2-inch-deep) slits into bottom and sides of pork. Rub garlic paste over roast, pushing into the slits and scores in fat cap. Place seasoned pork on a rimmed baking sheet; cover tightly with plastic wrap, and refrigerate 4 to 12 hours.
Preheat oven to 350°F with rack in lower third position. Unwrap pork shoulder. Place pork, skin side up, in a medium-size roasting pan; let rest at room temperature while oven preheats. Roast pork, uncovered, in preheated oven until a thermometer inserted in thickest portion of meat registers 200°F, 5 to 7 hours, tenting with aluminum foil after 1 hour and 45 minutes if needed to prevent burning. After about 2 hours of roasting, begin basting pork with pan drippings every hour. Transfer cooked pernil to a cutting board, and let rest 30 minutes. Reserve drippings in pan. Do not turn oven off. Carefully position oven rack in middle of oven.
While pork roasts, make the mac and cheese:
Bring 3 quarts water to a boil in a large pot over medium-high. Stir in 1 tablespoon kosher salt. Add pasta, and cook according to package directions until al dente. Drain pasta, and rinse well under cold water. Whisk eggs vigorously in a large bowl until frothy, about 1 minute. Whisk in cream, seasoned salt, black pepper, and remaining 2 teaspoons kosher salt. Stir in 5 cups cheese until well combined. Gently fold in cooked pasta. Spoon mixture into a 13- x 9-inch baking dish lightly coated with cooking spray. Sprinkle evenly with remaining 1 cup cheese. Cover and refrigerate unbaked mac and cheese until pork has finished roasting.
While pernil rests, uncover mac and cheese. Place on middle rack in oven; bake at 350°F until light golden brown on top, about 45 minutes. During final 15 minutes of baking mac and cheese, remove skin from pernil, and finely chop. Shred meat into bite-size pieces. Transfer shredded meat and chopped skin to a large bowl; toss to combine. Discard pork bone, or reserve for stock. Pour pan drippings into a fat separator or a measuring cup; skim and discard fat. Season pernil to taste with skimmed pan drippings. Divide mac and cheese among bowls; top with pernil.
Mac and cheese can be made up to 1 day ahead, covered, and refrigerated up to 12 hours. Let come to room temperature before baking.