Heat oven to 425°F; oil a rimmed sheet. Mix breadcrumbs with meatball water; gently fold in pork, egg, ginger, salt, and garlic powder. Shape 24 meatballs; bake 16 to 20 minutes until centers reach 160°F.
Meanwhile, simmer broth, soy sauce, vinegar, brown sugar, hoisin, and pepper flakes 2 minutes. Transfer hot sauce and meatballs to a 4-quart slow cooker.
Cover; cook on HIGH 90 minutes. Fold in pepper and celery. Whisk cornstarch with slurry water; gently stir into sauce.
Cover; cook on HIGH 30 to 40 minutes more until vegetables are tender-crisp, sauce glossy, and centers reach 165°F.
Turn off cooker; gently fold in sesame oil and peanuts. Serve about 3 meatballs per appetizer portion with sauce and vegetables.