I agree that Anders is the character that ties it all together. This would be a challenge to film simply because there are so many back stories. I like Aurora's idea for some sort of prologue introducing the locals and the summer people and maybe Anders Dad and his problem. The film could open with Anders returning to the Island as a drunk and more importantly as a narrator relating his memories of the people he knows on the Island. Different sights would prompt flashbacks with narration. The lighthouse would appear early on, in both summer and then winter. (cue ominous music
Once the setting and the characters are introduced then we could get on with the mystery.
What happened with Henrik and Bjorn in the boathouse could be done in a later flashback. That whole scene could be shortened and simplified. Hubba and Bubba get beat up, cut to Hubba and Bubba heading out in a boat "and nobody ever saw them again . . ."
I am for keeping the Spiritus (explained in a short flashback) and keeping the "don't drink the water idea." Simon's dialogue for explaining that has already been written. Standard horror movie stuff.
What I would pay to see would be Anders in Maja's bed drinking his "juice" and reading a Bamse comic book.