I disagree. Kyo being a sadist and a dom who loves sex and knows how to use her charms is part of her personality. It is how she was trained by her master and how she made her web of connections that brought her to be the emperor's affair/spymaster.
Just like the part of her that actually cares and love Catherine enough to try to make amends and change their relationship. That duality is one of the things that i like about her.
Also about Briala, it was obvious from the first time Catherine met the guard captain that she had the hots for Eleanor. And after they finally hit a friendship it became clear that Cat and specially Briala liked to dominate their partner while Eleanor liked to be dominated. From there it was not really a big jump for them to have that threesome.