In a medium bowl, combine beef, pork, eggs, Italian seasoning, garlic powder, celery seed, onion powder, smoked paprika, and oats. Form into 20 meatballs. Press 1 mozzarella ball into the middle of each of the meatballs.
Press the Sauté button on the Instant Pot®. Heat 2 tbsp avocado oil. Place 10 meatballs around the edge of the Instant Pot®. Sear all sides of the meatballs for 3–4 minutes. Remove meatballs from Instant Pot® and set aside. Add the remaining 1 tbsp avocado oil and sear remaining meatballs for 3–4 minutes. Remove meatballs from Instant Pot® and set aside.
Discard extra juice and oil. Add seared meatballs to a 7 cup glass dish. Add drained diced tomatoes.
Add 2 cups water to the Instant Pot®. Add trivet. Place the glass dish on top of the trivet. Lock lid.
Press the Manual button and adjust time to 20 minutes.
When the timer beeps, let pressure release naturally for 10 minutes. Quick-release any additional pressure until float valve drops and then unlock lid.
Skewer a meatball, a basil leaf, and a tomato slice on a toothpick. Repeat 20 times. Drizzle skewered meatballs with olive oil and balsamic vinegar. Serve immediately.