She will need another telepath or a special godclad to remove the mark in her mind. Even if they could, they would have to be extra careful not to harm her mind in the process.
Another advantage is that since the mark is in her mind, they can use it to communicate with her regardless of the distance. They don't need to use the mental probe ability to be in contact with her mind to communicate mentally with her.
So the pathway of the telepath is enhancing the pathway of slavery and vice versa. This kind of power is why they decided to drop the pathway of travelers for the pathway of the telepath.
They led her to the outskirts of town but still didn't show up to meet her. Instead, they made her wait there for some time while they made sure that there was no one else following them and that she didn't have accomplices.
