I am pretty sure Yvonne is supposed to die in every route (except her own, natch), since that's how it usually goes for a villainess in this kind of game.
Pretty sure we've already been told that this is not the case.
I'm pretty sure of that too. There's an "exile ending" after all. The one that Yvonne originally thought she was just on and was free of the heroine before Elsa chased her down.
Well, she did say the endings were not exclusive so there was still a possibility to pile more on the exile ending, but at the same time, she's also repeatedly confirmed her motivation for steering Elsa on a suitable route to be that this would free her of the villainess role. And once that's done, there's no reason for things to get any worse for her, even less for her to die.