Combine the brine ingredients in a large bowl. Add the chicken and marinate overnight, no more than 21 hours.
Preheat the oven to 350°F. Heat 1 inch of canola oil in a large skillet to 375°F. Set a wire rack over a baking sheet.
In a shallow dish, combine the flour, cornstarch, Old Bay, pepper, celery salt, onion powder, garlic powder, cumin, and paprika. Remove the chicken from the brine and dredge in the flour mixture, shaking to remove any excess flour.
Fry the chicken, working in batches so as not to crowd the pan, until golden and crisp on both sides. Transfer the chicken to the prepared wire rack and baking sheet and finish cooking in the oven for 5-10 minutes.
In a large pot, place the potatoes and a generous pinch of salt. Cover with cold water and bring to a simmer. Cook at a simmer until the potatoes are fork-tender, about 15 minutes. Drain the potatoes and return to the pot. Add the butter, milk, buttermilk, salt and pepper. Mash until smooth. Fold in the jalapeños.
In a large bowl, make the slaw by whisking together the mayo, mustard, lemon juice, sugar, salt and celery seed. Fold in the cabbage, carrots and onion. Season to taste.
To serve, divide the chicken and mashed potatoes between plates. Serve with coleslaw on the side.