I think Evita DID get it wrong and OReilly WAS the dying half. The loophole was that, as Tandy said just before they drained out the Darkforce energy that was welling up, Ty and Tandy are basically two-halves of ONE PERSON (in the same way that you need both Yin and Yang to have a complete person).
Its also worth noting the shoulder markings that each sacrifice had in all the stories. Its why Evita was so certain that Ty would die; because Ty got marked on the left shoulder. Then, just before the end, Tandy got marked on her right shoulder (Yang to his Yin).
But Detective OReilly had a mark on BOTH shoulders the entire time before either of them (the two white stripes on her jackets shoulders). Likewise, her life/story got intertwined with both Tandy (the first investigation) and Ty (taking down Conners) over the course of the series. We also followed her throughout last episodes recounting of the heros journey as she went through her own dark night of the soul.
OReilly was the other half of the pairing that died just before the Ty/Tandy half fixed it all. Now shes some sort of undead monster (Im pretty sure OReilly is dead and that just the darkness wearing her skin). Which also fits with her being shown in the previous episode at the point where the priest explains that some people who go through the heros journey become villains.
What makes that more interesting is that Ty/Tandy think theyve pulled off a screw destiny, so the reveal that OReilly is some sort of revenant opposite number to them (perhaps needing to kill both of them so it can be reborn) will be the exact sort of setback for them youd expect and also provides a solid way to pull Evita and the voodoo elements back into the narrative next season.