So why did Rebecca............

It's a pretty common reaction to this type of situation where the abuser is in a position of power or seniority.

Rebecca said herself that she was obsessed with him. She may have been in denial about what happened and writing to him in a friendly manner was her way of not having to see herself as a victim and instead convincing herself that it was a positive sexual encounter, so that her image of him could remain untarnished.

Or similarly, by writing in the hope of seeing him again, she could have been hoping that if she was the one to initiate their next encounter then she would be the one in control - again meaning that she wouldn't have to see herself as a victim and would have a positive memory to replace the traumatic one.

I think it was just a red herring to the audience to help convince us Paul was innocent

It's called a "plot hole."