This is the explanation (MAJOR SPOILERS)

1. the only dream sequence is the one with the house of lemmings when he crashes his car. This is also set up as a red herring so that we wonder whether he's dreaming the rest of it.

2. the spirit of Alice takes over Bénédicte, which is why B sometimes becomes quite nasty yet can't remember it later (eg when she hangs up on Alain)

3. Alice takes over Bénedicte every now and then, eg beside the lake (when she knows what Alice said in the lab... though we may wonder at the time that perhaps Bénédicte saw the scene via a webcam... this is another red herring).

4. Bénédicte DOES have an affair with Pollock but only when possessed by Alice's spirit. She then returns to Alain, gives him the key, and appears as both B and Alice, so that he kills Pollock. She wanted to see him croak, remember?

5. Alain comes in and kills Pollock, and Bénédicte is now free of the spirit of Alice, demonstrated by the simultaneous death of the lemming. Bénédicte does not remember any of the possession by Alice. The lemming, rather than having Alice's spirit, represents the passing of Alice's spirit to Bénedicte (and clearly has some of Alice in it, when it bites Alain).

6. What we do not know is whether Alain knows
i. that his wife indeed became possessed, and asked him to kill Pollock; or
ii. that his wife had the affair as Bénédicte and has now forgiven Alain; or
iii. that he (Alain) is having another delusion like after the car accident and the "suicide" of Pollock was another coincidence or a premonition.

We know the answer is (i) but we don't know what Alain thinks. Alain is as confused as the viewer is. Which is quite a neat ending.

See Dominik Moll's interview on the DVD special features (on the R2 release in any case) for confirmation of this. More or less.