What caused smooth homemade nutella to become grainy in refrigerator?
You're right, it's the coconut oil. Notice how shiny the batch was fresh? That's the oil separating. It would need constant stirring while cooling to keep droplets from forming. Steady cooling  - I got that graininess in ice cream once by freezing too quickly from room temp.

Got an ice cream churner around to try with just fridge temp?