Ironically Diavolo and Lucifer are the two whose dialogue I took straight from the game instead of changing it, haha. As for the MC being calm, I imagined it to be like at first you just kinda think you're dreaming/are in denial, if that makes sense? Once Levi tries to kill you, though, that's when the shock and realization truly hits. In all honesty it is pretty difficult to have proper reactions in the very beginning when the player's personality isn't put in, so I apologize.
Still, I appreciate your feedback a lot! The Lucifer/Diavolo and memories situation are both valid points and I'll do my best to fix it in Version 1.0 to have it seem a little more organic :)