I want to enter the New Directors showcase (category 1) for a TV spot I directed. Can I submit a directors cut or does it need to be the official spot that aired?

Up to you what you submit. Keep in mind, that if selected, your work would be shown at DGA and publicly on the SHOOT NDS Web Reel for 1 year. Meaning, if your Directors cut doesn't have proper clearances could be a problem for you.