Heat oven to 425°F; oil a rimmed sheet. Soak oats in meatball milk 5 minutes; gently mix with beef, egg, meatball salt, and pepper. Form 24 meatballs; bake 16 to 20 minutes until centers reach 160°F.
Meanwhile, simmer broth, pepperoncini, brine, 2 tablespoons butter, dill, chives, garlic powder, and onion powder. Transfer hot mixture and meatballs to a 4-quart slow cooker.
Cover; cook on HIGH 2 hours, gently turning once. Whisk cornstarch and buttermilk powder with slurry water until smooth; fold into sauce. Cover; cook on HIGH 15 to 20 minutes until thickened and centers reach 165°F.
During final half hour, cover potatoes with cold water by 1 inch; add potato-water salt. Bring to boil; simmer 15 to 20 minutes until tender. Drain; return to warm pan 1 minute and mash by hand.
Warm potato milk with remaining butter; fold into mash with potato seasoning salt. Divide among 6 plates; add 4 meatballs, gravy, and a softened pepperoncini or two each.