Heat oven to 400°F and brush a rimmed baking sheet with 1 teaspoon oil. Soak torn bread in milk for 5 minutes, then mash lightly with a fork.
Mix soaked bread with pork, veal, egg, Pecorino, 1/4 teaspoon salt and pepper just until combined. Shape into eighteen equal meatballs, space on the sheet and bake for 18 to 22 minutes until 160°F in the centers.
Meanwhile, heat remaining oil in a skillet and soften onion for 5 minutes, then add garlic for 1 minute. Transfer to a 6-quart slow cooker with tomatoes, water, oregano and remaining salt.
Add cooked meatballs, cover and cook on low for 3 hours. Turn to high and lift meatballs to a covered heatproof bowl while starting the pasta.
Stir hot broth into the sauce and add dry spaghetti in several crossed layers, pressing it below the surface. Cover for 15 minutes, stir carefully to separate the strands, then return meatballs and cover for 15 to 25 minutes more until the spaghetti is tender.
Stir gently from the bottom once more, check that the meatball centers are 165°F and switch off. Divide spaghetti among six bowls and add three meatballs to each.