I think Ocoho is brainwashed exactly because she has talent as a medium, high spiritual sensitivity is a great talent but if not well developed, especially in the area of shutting your senses can leave you susceptible to "invasions", i mean it has to have a reason why Ocoho is the one they are using and i highly think it is that.
With Ocoho is one more person that don't mind Melie fits and we now know why Boobrie likes "badass" Melie so much, and i am surprised that the other Melie would go so much out of her way to protect Ocoho that way ( it was the other personality on the latter half of the chapter, no? )