i think you dont understand the types of jutsus used

Orochimaru uses alot of seal based jutsu so that makes it easy for Itachi to copy all his jutsus, Orochimaru spent his life accumilating jutsus and do you think hes going to let Itachi copy them all?

as for people who say that since Itachi is feared by Orochimaru that he can easily beat him - havent you ever feared something like a spider? doesnt mean you cant kill it does it, the capicity to kill it is still there.

Jiyaira and Orochimaru use completely different fighting styles, stop thinking that everyone is exactly even against everyone else.