I just came back from a reddit discussion on this series and the consensus was, this twist is so good and someone even mentioned it made the first part now understandable. What am I missing??
As I see it, the point of this arc is to explain Elsa becoming infatuated with Yvonne, to the point she thinks of Yvie as her "special person" as opposed to one of the male leads the game initially had destined for her.
It is hinted at by the cloaked woman that Yvonne had forgotten about the time she and Elsa spent together as children, though the mechanics of that are as of yet unclear - my suspicion is that what has so far been referred to as the "villainess system" is a high-tier Mare that can seal off certain memories and command its "vessel" but isn't able to manifest like the Mare possessing Elsa way back when did.
Drinking the succubus potion possibly awakened a power said Mare is unable to control since it's goal is to "keep the villain parameters in check" - and while it is highly powerful, it is not all-powerful, as we've seen in the scene where the cloaked woman (who I suspect to be another incarnation of Elsa) is able to break it.