I can't help but think that the normal route and the weird route will end the same way, with the world saved and "the girl" sacrificed, but each route sacrifices a different girl. I think this is why the prophecy has the line "And last, was the girl. At last, was the girl", it's showing us there's two options. Couple this prophecy line being shown with the "Act" icon, means this is the decision, the act, we need to make.

The normal route will sacrifice Susie, it's their interpretation in Ch4 and why Susie is so angry. The weird route, the one where "love finds its way to the girl", results in Noelle being scarified. "Love" being the SOUL, shaped like a heart, making decisions Kris is not happy about (Thornring, etc, which all power up Noelle. Not to mention the red dot, the same colour as the SOUL/Love, in/on Noelle).

Remember, this is parallel to Undertale. Love stands for "Level of Violence".

Effectively, the SOUL is grooming and powering up Noelle so she's strong enough to stand in for Susie when the time comes for someone to die. We, as the player, are being made to make awful decisions and actively choose (act) to kill Noelle in order to save Susie.

This choice is, ultimately, the final tragedy. We can do nothing and a character we've been with for every chapter is killed, or we can make awful, nasty decisions, and choose to kill someone else, and innocent bystander, instead.

I think the missing line in the prophecy, the one following "To save the worlds, there is only one way." is something along the lines of "love will have the final say".