Susie is not a part of the prophecy.

The prophecy was originally supposed to be Kris, Noelle, and Ralsei. Noelle was supposed to go with Kris into the closet and fall into the darkworld, starting their adventure. But Susie is a rogue element, and was not predicted in the prophecy. She can ignore fate entirely and insert herself into the story and fuck with the prophecy. It's why the girl is depicted with a cloak and holding a sword, rather than as a big dino with an axe. Susie is mistaken thinking "the girl" is her in the prophecy - it was SUPPOSED to be Noelle, but she ended up being taken out of the prophecy by Susie's fate manipulation.

This is why Gerson mentions that Susie can rewrite fate with hope, with her own willpower. The broken prophecy passage is that the girl dies, but it will never come true now because Noelle isn't with them and Susie isn't a part of the prophecy.

Susie IS the key to all of this, and the weird route will be you taking the original course of the prophecy, while the normal route will be having Susie fuck with it to get a better ending.