Oh, no, don't let it be like Lost!
Glad to see I am not the only person overthinking this 
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The Dam is certainly one of the central characters, I think. Or it's water.
I actually think that the only people who reacted somewhat 'normal' to the reappearance of a dead person are Lena and Simon's fiancee IMO - screaming heeby-jeebies are perfectly appropriate in those circumstances.
Like the idea of a suicide pact gone wrong.
The stabbing: she was quite on top of things. Why did she not run away when she had a chance?
The twins have not swapped places, but was Camille complicit in allowing Lena to bunk off 'sick' in order to meet loverboy?? Even though she was a bit in love with him too?
I think Victor DID make the bus sverve - the accident just did not look 'natural' ie a bus going in to a bend too fast would've gone over the edge by its tailend, rather than go straight off.
Victor is spookiness personified
Mr Costa. Hm. Do we know who the woman was he burnt? His sister? Wife? Lover? Secretary? Why burn all the photos??
Tous les questions!!