Heat oven to 425°F; oil a rimmed sheet. Soak breadcrumbs in milk 3 minutes. Gently mix with beef, pork, egg, salt, pepper, and onion powder. Form 24 equal small meatballs.
Bake 15 to 18 minutes until browned and centers reach 160°F. Meanwhile, warm jelly, tomato paste, vinegar, broth, and Worcestershire in a saucepan, whisking until smooth and steaming.
Pour hot glaze into a 4-quart slow cooker; fold in hot meatballs. Cover; cook on HIGH 2 hours, stirring gently after the first hour, until centers reach 165°F and glaze coats meatballs.
If glaze is too thin, uncover for the final 10 to 15 minutes, stirring once to prevent edges catching.
Switch to WARM for serving only if food stays at least 140°F. Keep covered between servings; serve about 3 meatballs per appetizer portion.