Why was her mother there?
I think that most of the events of the film heavily indicate that the "fantasy" elements, within the context of the story, were meant to be completely real and not Ofeila's imagination.
I also know that Del Toro has outright stated that this was his intention.
The biggest thing that works against this is the fact that Ofelia's mother, who died because of her refusal to believe, is there in the afterlife/spiritual kingdom Ofelia arrives at in the end.
Why would she be there if she didn't believe at all?
+++by His wounds we are healed. - Isaiah 53:5+++