Authors don't always have full control of their story line sometimes their bosses tell them to change certain parts to fit budget or people's views. So if less people are reading/buying the story then they have to rap up the story which causes them to cut out a lot of important parts to finish the story.
As for what the elf said here's what I understood: the elves rejected those who prophesied because the Gods and their prophecy were a vague thing which could be manipulated by the will of the one who got the prophecy. Granted they have enough will power and belief. So kinda similar to what we in modern times call manifestation. And that Sefal made the false prophecy into a real one by his own belief.
Hmm. I think while Caius was taking the most from his father that was probably because he was stronger but in no way was it meant to cause chaos in their kingdom. I think he was just naturally going to be a strong king and his father would've passed away. That part is true. Sefal tried to manipulate the prophecy by sending away Caius by saying that if he stayed and married into the same species the kingdom would perish. I think that is the lie. But ultimately the prophecy came true even if he thought he manipulated the prophecy but in the end it did come true which I think Sefal realized at the end there. So yeah vague. This is to the best of my understanding i could be wrong in some parts.
You are also probably right in that the author may have fumbled a bit. We never know what's going on in the writer's room.

How is the story this long but stuff isn't set up well? And I'm confused; Sefal stated Caius was sapping his dad's strength, but the elf said he fabricated the prophecy, so was that first part true or not? And LITERALLY in five seconds the issue is resolved; Caius doesn't get to feel broken and Sefal is begging for forgiveness. This is the same rushed storyline as Julg! Has the author ran out of ideas?