As i said just because a ninja hasnt used a technique against a another means nothing, merely kishi's doing. Example if jiraiya had used the swamp of the underworld against nagato's summonings he wouldve been able to get into sm alot easier. Similarly if he had used the toad mouth technique, agains the paths from the start, all 3 path do not have a technique to destroy that toad stomach, yet jiraiya didnt use it. Not because it wouldnt be successful because it would, but because its all kishi's doing.
Secondly it wasnt itachi's fight. He was in the background for majority of the fight. Also you say genjutsu doesnt work on the dead and yet ka a genjutsu worked on itachi or am i missing something. Also genjutsu has already worked on the paths although it was a sound genjutsu.
Genjutsu sure hasnt killed betwenn high level shinobi's but you see itachi couldve killed oro. He couldve let deidora blew himself up. Couldve killed kakashi, couldve killed sasuke, couldve killed naruto. Also the 2nd mizukage who is main fighting style being genjutsu killed the mu, although both died.