Ghost or imaginary

I think Mirren, Johnny and Gat are Cadence's imagination. Firstly, I don't believe in ghost. Then in the book, the author writes that, Cadence got injury in that accident and had mental problems. She can't remember what happened in that accident. So in this condition, I think Cadence may have imagination due to the injury. When Cadence set the fire then she found other liars were still in the house. She was extremely sad and scared, and felt guilty. Then want to make this family be better but finally she destroyed it. I think this kind of serious sadness may cause Cadence have imaginary of liars. Because she didn't want them be died. She lived in her imagining world, she imagined that they were still alive, and the whole family relationship got better and better which is what all four liars hope. And the last reason is that, in the book, there are only conversation between Cadence and liars. But liars never talked with other family members in summer seventeen which means other people can't see them. And when Cadence once asked Carrie about how was Johnny. Aunt Carrie didn't know what she was talking about. If they are ghost, everyone in this island can see them. But they couldn't. So according to above reason, I think Johnny, Mirren and Gat are imaginary.