What if the dog sightings were inaccurate? I believe the owner of the caravan park was the one who found willow and the phone etc. The cctv also at said park happened not to be working. If these statements are wrong it throws the timeline by anything up to a good hour when the school was called.
What if the boy was babysitting because grandparents away for the night, dad staying with girlfriend? Grandparents return on the morning ferry and find her missing & the boy unable to explain where she is?
Just to chip in, what are the police looking for on the beach a day or so after the event? High tide is every 12 hours and google maps shows the sea opposite the house right up to the wall.
Whatever they are looking for is heavy and not likely to be washed away by the tide?