@Nomenclature
You may have not realized it, but you just spoiled right now. Not big because we already had clues about that, but still a bit.
You have to remember that the premise of the manga was clear for the first chapter: it was to follow Shiraori, trying to survive in this unknown and cruel world. Not more, not less. Now, as I said, they already were clues (or even obvious sign if you will) that she was gonna have a bigger part in this world (whether it's with D's phone call or the demon queen), but so far the story was still following the premise it began with.
It's clear from the start that the manga doesn't follow the same storytelling that the LN, which is good because it not a manga
retranscription but a manga
adaptation. That means that, in the process, some things were bound to be changed.
You're free to like the LN more, but if you were expecting the exact same story that the LN, just reread the LN directly.