Good explanation thats the kind of answer i look for, only thing i think is probably not right is that kurenai would not be useless if she was up against Jiraiya who is useless at genjutsu unless in sage mode
she is useless against itachi, that's certain, it already happened
about jiraiya you are right.
but back then when kisame+itachi vs. kurenai+asuma happened kurenai tried to take both of them into an illusion.
but failed duo to itachi.
the moment when itachi realizes that kurenai used a genjutsu he'll step in.
however if everything fails and kurenai manages to take jiraiya out with a genjutsu (not likely though) itachi cannot win against the other 4
the only way they lose is that