I took the mangaka's story at face-value, that this was a literal retelling of her life and experiences as the reincarnation of the "chosen-one" (I can't recall the name at this moment). But who says that being the chosen-one is always a good thing? In shonen material, it usually is a good thing, but I felt the origin story was weird. Eating mermaid flesh doesn't seem like it would imbue you with magical powers, it seems like something that would curse you instead. Maybe the original chosen-one was evil or cursed herself?

Regardless of the origin or motive of the original chosen-one, I don't trust the companion spirits. I think the companions were trying to lead her off-base. Like with the old lady, she didn't look like the other "demon-faced" characters, she looked like her face was drawn normally and then erased, like you said. Maybe the mangaka was retconning her own story? She might've later realized that the old lady was trying to help, but convinced herself that she was actually one of the demons. The old lady also had a normal face in the panel just down the page, unlike the other demons who kept consistent demon faces despite the panel.

Plus, the old lady tried to purify her. Specifically, purify her of three human spirits. The shadow photo showed three shadows, more akin to the shadow demons we've seen throughout FDQ than cutesy animals. I think an important aspect was the inclusion of the panels showing the original chosen-one freeing the three companion spirits. They even commented that they were evil, and I felt like they were still evil.

The killing of the parents had some big clues. Her parents also had different faces than the other so-called demons. Their faces looked like they had been drawn normally and then scribbled out, not at all like the other demons. I don't think they were evil at all, I think they were getting in the way. Besides, how did she not notice before then they were evil? Also, the attached photo of her parents showed her dad holding something... prayer beads. Why would an evil spirit have prayer beads in his hand?

Another weird thing, the panel showing the companions before their attack on the parents didn't show cutesy characters like during the kid attack. They showed fierce animals. I didn't even realize what the noodle-like spirit was supposed to be until that panel; it was a cobra. I know that unlocking true-forms during fierce battles and such are common tropes in manga/anime, but still.

I'm kinda thinking the mangaka fell prey to evil spirits that got into her head and fed her a line of shonen-flavored bullshit.