Of course Itachi would win, do not take his fight against Sasuke to be the best he can. If he wants to go for a kill, besides Madara and a couple few (who are alive, pls don't mix the dead) no one would stand a chance.
Itachi is a genjutsu MASTER, he is very proficient in using tsukiomi. And no, Naruto is not immune to genjutsu, it was not proven so far so everything you say is just an assumption. He can't because he can not merge with the 2 frogs, and the kiuby will not help him break free.
Itachi's susanoo is indeed the ULTIMATE DEFENCE, with the shield and the sword Itachi is pretty invincible (even if it is not for a long period)
And about amaterasu, we don't really know what effect it has on Naruto in SM, considering he can control the space surrounding him to a certain degree he could be able to avoid it.
And also do not get carried away by Naruto's fight with pain, he was able to go in SM 3 times... against Itachi he will not have this chance. (It would be unfair, if you consider this then also take away itachi's ilness)