How Much CC Knew About Geass

You should not consider her as some sort of Gendalf level wise and super capable being. She is but a girl who lived a long life.

Sure, she has some supernatural powers, but other than that she is very ordinary girl and what she knows and what she can do is limited.

She does not understand how geass works, nor does she understand what to do if it goes awry.

In her flashback she literary resisted Code of Immortality taking over her body. She even said, 'do not invade me'. She clearly has no idea what that power is, why she has it or how to live with it. She learned all she knows from experience. No one provided her with any explanation or manual on the powers she has.

Same how Lelouch had to learn how his geass worked in an experimental way by geassing his teachers and other students to see how long it will last and so on.

A bonus: some about CC's Flaws

Using Lelouch's credit card and being stubborn and disobedient at times, like that that episode with the bed.

However, her flaws are somewhat inconsistent with her character, you would think they are more of accidental behavior, caused by the circumstances, than her actual self.