For the pulled pork: Combine the pork, barbecue sauce, brown sugar, chili powder, chipotle powder, lager, 3 tablespoons vinegar, 2 teaspoons salt and the onions in a 6-quart slow cooker. Cook on high power for 6 hours.
With a slotted spoon, transfer the pork to a cutting board and shred using two forks. Transfer to a bowl and add enough of the liquid from the slow cooker to generously moisten. Stir in the scallions and a splash of vinegar and season with salt to taste.
For the fried shallots and chiles: Set a cooling rack over a baking sheet and set aside. Add enough oil to a medium saucepan to fill by 2 inches. Over medium heat, bring the oil to 340 degrees F. Meanwhile, put the egg whites in a medium bowl and whisk with a fork until frothy. In a separate bowl, combine the flour with the chili powder, cumin and some salt and pepper. In batches, add the chiles and shallots to the bowl of egg whites and toss to coat. Remove from the egg whites, allowing excess liquid to drain off, then place into the flour. Put the coated chiles and shallots onto the prepared cooling rack. Fry in batches until golden brown, about 2 minutes. Remove to a paper-towel-lined baking sheet and sprinkle with celery salt.
Serve the pulled pork topped with fried shallots and chiles, or assemble into sandwiches:
Add the pulled pork to 6 slices of bread, then top with fried chiles and shallots, pickles and the remaining 6 slices bread.