Cut spaghetti squash in half lengthwise; discard seeds. Bake squash, cut sides down, on a lightly greased baking sheet at 400° for 1 hour. Cool 10 minutes. Combine ground chicken, green onions, parsley, 2 Tbsp. freshly grated Parmesan cheese, and salt. Shape into 24 (2-inch) balls. Bake at 400° for 30 minutes. Scrape inside of squash to remove spaghetti-like strands (about 6 cups). Cover and chill squash and meatballs up to 24 hours. Cook garlic in hot olive oil in a Dutch oven over medium-high heat, stirring occasionally, 1 minute. Add pasta sauce, diced tomatoes, and meatballs; bring to a boil. Reduce heat to medium-low; simmer 10 minutes. Microwave squash, covered with plastic wrap, at HIGH 3 minutes. Stir basil into sauce; serve over squash. Top with Parmesan cheese.