I've finally figured out the circumstances under which Nev would have become the WoL. In her default canon, Ireneve's family is loosely tied to the Dzemael line through a knighthood/marriage. However, when the family branched off, no longer continuing the knighthood, they lost a lot of the Nobility that would have been afforded to their otherwise Noble name.
So, in this AU, the Knighthood was upheld. Her family never left Ishgard. When the freeze came, it took much of their livestock and workers, but not them. All the while, Ireneve has been brought up being told she would become a knight, training always with this in mind. She would have also had to practice her lying a lot more.
Despite the bad blood between their families, she would connect with Artoirel much sooner in life. And, later, when she is well on her way to becoming a Templar (claimed by them, rather than the Dragoons), she would learn of the corruption of the church. And she would want to do something about it. Because she's still Nev.
The Fortemps men would stop her from getting herself killed. She would have to leave Ishgard to protect her family and pray that the Fortemps could do something in her place. Meanwhile, Yda and Papalymo find her, very disgruntled and probably slicing up a tree, and recognize what her uncle called clairvoyance as the Echo.
From there... the rest would go as it does in the game, for the most part!