For the pimento cheese and buttermilk ranch dressing:
To make the pimento cheese, mix ingredients through celery salt in a large bowl, using only 1/2 cup of the mayonnaise. Season with salt and pepper to taste. Can be made 3 days ahead.
To make the dressing, combine remaining 1/4 cup mayonnaise and sour cream. Stir in shallot, garlic, lemon zest, and mustard powder. Slowly whisk in buttermilk. Next, whisk in 1 to 2 tablespoons lemon juice (until dressing reaches desired thickness) and the sherry vinegar. Finally, fold in herbs and season with black pepper. Cover bowl and set dressing in the fridge to chill for at least 20 minutes, but preferably overnight.
For the fried pickles and the assembly:
Set up three shallow bowls or baking dishes: In one, pour the flour. In the second, beat together milk, egg, cayenne, and a splash of pickle juice, In the third, mix together the cornmeal, paprika, salt, and pepper.
Thoroughly dry pickle slices with paper towels, then dredge the slices by transferring them through the shallow dishes: flour, egg mixture, cornmeal mixture.
Heat an inch of peanut oil in a Dutch oven over medium-high heat until it reaches 375° F. Working in batches to avoid crowding the pot, fry the pickle slices until golden brown, about 1 1/2 to 2 minutes. Transfer to a sheet tray lined with paper towels and immediately season with salt.
To assemble the sandwiches, spread pimento cheese on the buns or rolls. Top with fried pickles, then drizzle with the buttermilk ranch.