Preheat oven to 250 degrees and remove and discard plastic wrap on pork. Place trussed, butcher-paper-wrapped pork in a roasting pan and cook for 2 hours per pound. Remove from oven and let cool for 10 minutes. Cut off and discard netting and butcher paper.
For the cheese sauce, in a medium nonstick frying pan over medium heat, melt butter. Add flour and whisk until smooth. Add cold milk, whisking constantly, until smooth. Lower heat to low, add salt and cayenne pepper and whisk to combine. Add cheese, in batches, and gently stir, until cheese is melted. If sauce is too thick, add milk, 2 tablespoons at a time, until desired consistency is reached. Transfer cheese to a measuring cup or pitcher and keep warm until ready to use.
To assemble, in a large serving bowl, place a layer of chips and dress with 1/3 of cheese sauce. Add another layer of chips and sprinkle with beans, tomatoes, corn, and pork. Dress again with 1/3 of cheese sauce and top with guacamole, sour cream and salsa. Garnish with remaining 1/3 of cheese sauce, jalapeños, cilantro, and green onions and serve immediately.