C.C. was helping them with their plan about ten seconds before she agreed with Lelouch and long after Marianne died.
She was not lead along by Marianne. She created a contract with her that quite simply said "Let's destroy God", which would have made her death, the single thing that she had been seeking for hundreds of years an impossibility. Tell me, does this make any kind of sense?
I understand what the world of C is now, yes, but there are many other things that need to be questioned.
For example, Charles wanted a world without lies, a world without masks. What did he do to achieve his goal? Put on a mask and hid his true self from Lelouch and Nunally, while simultaneously killing his brother for lying in the exact same manner he did. These actions don't follow anything close to a logical order.
The explanations were confusing, and the character's actions made no sense. They created a twist that is too elaborate and complicated for them to handle.