The best cider I've ever made was primed with juice, not sugar. I used a little over 5 oz (by weight)/5 gallons.
As far as cinnamon, I'd put some cinnamon sticks in a mason jar, then pour in enough vodka to cover the cinamon. Maybe break the cinnamon sticks into 1 or 2 inch pieces. Then at bottling, take a measured sample of cider and add the spiced vodka to the cider to taste. Once you know how much spice suits your taste, scale that amount up to the full batch size.