Cook udon in a medium pot of boiling salted water according to package directions. Transfer udon to a colander to drain; reserve cooking liquid in pot. Transfer udon to a large bowl and toss with 1 Tbsp. oil. If using eggs, cover pot and return cooking liquid to a boil. Add eggs and cook at a low boil until soft-boiled, about 6 minutes. Transfer eggs to a large bowl of ice water to cool, then peel and reserve. Heat remaining 2 Tbsp. oil in a large pot over medium-high. Saute mushrooms and 1/4 tsp. salt until lightly browned and moisture releases, about 5 minutes. Stir in garlic and ginger; cook until fragrant, 3060 seconds. Stir in miso, then add chicken broth. Stir in kombu, if using, soy sauce, and remaining 1/2 tsp. salt. Cover, bring to a simmer, and cook 10 minutes. Add meatballs and carrots. Cover and continue to simmer until meatballs are just cooked through, about 10 minutes. Remove kombu from broth and discard. Stir in bok choy and vinegar, if using. Place fish on top of stew, then cover and cook 3 minutes. Gently fold in shrimp (try to avoid breaking up fish) and cook, covered, until shrimp is pink and fish is opaque and cooked through, about 3 minutes more. Divide stew among bowls. Slice eggs in half lengthwise and top each bowl with 2 halves. Sprinkle with scallions. Serve udon in large bowl for sharing alongside.