Call me schizo, but I don’t believe for a second this is actually Fiona. It wouldn’t be a stretch to assume most of the pictures being posted by the account were at some point sent by Fiona to Bella (and possibly others) when this was initially going down. Also, the username selfie has no time stamp and is possibly months old, she talks in third person multiple times, Grant supposedly hasn’t heard anything from Fiona recently, and that audio honestly sounds like it was spliced from other recordings.
Ontop of all that, Fiona was allegedly just sent to the psych ward from having too much attention online and had her internet privileges taken away by her parents. So, not only did her parents supposedly give her internet access back immediately after she gets out of the hospital, but she also decides to draw a bunch of attention to herself online again, when she was just hospitalized for that very reason.
I really don’t think her just being a dumb autist explains most of this.