Dei'arc = Demonarc Unlike your null, they can shift perfectly. They pick people that are known to be deceased, for a reason that will be explained later on.
Let's see if I can summarize: Dei'arc Umbra willingly taking the form of the living means "kill me, I'm infected with a Nihiliod" So when Nyctores took on Ignis's form she was discretely asking someone to kill her without the Nihiliod knowing.
Ok Nevaraas there's two things to react to: Joe in the colony asking around if anyone saw Ignis or someone walking in on Lotus and Spear reparing the fighter.