I know the assumption she makes is that she took over the body after the original Shelina died from neglect. But based on her first conscious day in Shelina's body it didn't seem like the neglect was life endangering like the type she would experience later on. At the very least she woke up in a body that was bathed and fed. I feel like she is the real Shelina. It was hinted at some point that her pacifier is an artifact. I'm starting to believe it was the pacifier that made her recall her past life as Hella and that she wasn't "conscious" right from birth because she either didn't have the pacifier or it hadn't activated before she woke up in ch 2.
P.S: I don't read the novel and I don't wanna know any spoilers from it.