Agreed. This is currently the only rule book. The Red Book of Magic is borderline.
My version is the rule for all subcults, whatever the main cult.
Yes, there is a big advantage of being initiate of odayla as being a subcult of Orlanth compared to Odayla as a standalone cult. In addition, IIRC, you can be a shaman of Odayla, meaning you can be a shaman of Orlanth (through the subcult). I think it normal, as the cult is larger and tge god more powerful.