Okay so I think the missing piece of the prophecy we don't have is about Ralsei. I think there are few possible outcomes but all of them ultimately are about Ralsei not being around anymore. Either he has to sacrifice himself, either he becomes a Titan, or maybe Castle Town has to be sealed in the end, or maybe Ralsei just has to spend forever alone in the Dark World.

Chapter 3 spends a LOT of time making Ralsei alive. He gets to have fun. Play games. Experience joy with friends. He even, briefly, has mini-Kris gets to make choices. Decisions. Before hand he was fine being a "non-existent" he was doomed to death or being alone regardless. Then he made friends. Further reinforced by his friends finding furniture for him. He doesn't value a room. He won't get to keep it.

By lying with omission, he wants to protect the others from the fate he alone has to have. Knowing that specifically a prophecy saying he must perish would cause the reaction it most likely has for Susie and Kris, he wants to avoid this at all cost. If they don't know they can't stop it when it comes time. Chapter 4 has Gerson repeatedly make statements that allude to the prophecy being "up to interpretation" and that it can be changed. Meanwhile Ralsei is adamant that what it says will happen regardless.

However, we already see a flaw. The Girl does not match up with our girl. A sword. An axe. Refuses to wear ribbons. Something is already incorrect in the prophecy. I'm not sure where Toby will go with this because I truly don't think the "true ending" will be "the goat kid dies again" but I also don't think we'll get a scenario where any of our heroes come to pass. If I were to guess, the normal route will have our heroes find some way to save the day, most likely with a talk-no-jutsu on Ralsei at the end.