Prepare Shrimp Stock: In a saucepan, sauté the shrimp heads and shells in a bit of oil over medium-high heat until lightly browned. This brings out more flavor than simply boiling them.Add a bay leaf, a generous pinch of salt, some ground black pepper, and 6 cups of water. Bring to a boil, then strain the broth. Set it aside. Brown the Pork: In a large wok or sauté pan, spread out the meat evenly. Brown it over medium-high heat, stirring occasionally, until it's cooked through and nicely browned. Lightly season with salt, then transfer to a plate.
Cook the Remaining Proteins: Add the shrimp or fish balls and toss for about 30 seconds; they are pre-cooked and just need a quick heat-through. Add the shredded chicken and shrimp. Cook briefly until the shrimp turns pink. Transfer them to a plate.
Sauté the Vegetables: Sauté the carrots, green beans, and celery until they become tender-crisp, for about a minute. Add the cabbage and continue to stir-fry until the cabbage is slightly wilted. Set them aside.
Sauté Aromatics and Mushrooms: If needed, add more oil and sauté the garlic and onions until softened. Add the mushrooms and sauté them for about a minute.
Prepare the Sauce: Add about 5 cups of the shrimp stock, reserving 1 cup if needed later. Stir in the dark soy sauce, oyster sauce, fish sauce, and ground pepper to taste. Bring it to a boil, and taste to adjust the seasoning as necessary.
Cook the Noodles: When it reaches a boil, add the rice noodles and allow them to soak up the liquid. Use two wooden spoons to gently toss the noodles until they turn translucent and tender. Add more of the reserved liquid, as needed.
Combine and Serve: Return the cooked proteins and sautéed vegetables to the pan. Toss everything together until thoroughly mixed.Garnish with sliced scallions or celery leaves. Serve with calamansi or lemon wedges on the side.