Volume 34 of the manga gave it that the official spelling is Simon Cozzato. The “Cozart” spelling was the spelling the English-speaking fandom (especially at hitman_reborn@LJ) agreed upon based on the ‘kozaato’ katakana. I’m happy to finally have an official spelling. Enma’s surname, however, stays Kozato.
All of this information is with thanks to stradas who gave a picture of her tankoubon.
Information for Enma is thus changed to reflect this.