Regarding the “Younger, more beautiful queen” and the Rule of Three, I think you overlooked Sansa. Cersei definitely viewed Sansa as a threat, which is why she treated her so poorly (we just didn’t have the benefit of Cersei’s POV). Sansa (duck), Maergery (duck) and Arianne (goose). The head-fake may be that Cersei might not perceive Arianne as beautiful, and therefore not a threat.