Just caught up with this. Absolutely extraordinary film, and DH really loved it too. We are still talking about it several days later.
I think it was filmed in that very realistic style because it wanted to show how messy real life can be.
I definitely don't think she killed him. He was a long term depressive who was trying to come off his meds, he had great guilt over his son's accident, he felt he was a failure as a writer, especially compared with Sandra - the plot twist he had been unable to make anything of after struggling for years, she took and made into a success. In the recording of the argument he ended up beating himself up. At the beginning of the film his wife is emotionally stronger than him, more successful, she's fed up and unhappy because he's brought them back to his home town and it's not working out and costing them money they don't have, now she's sitting downstairs flirting with a young woman, BAM!
I think if she had killed him (and it would not have been easy as he was a big powerful guy) she wouldn't have just left the body lying there for the son to find, and she would have made up some sort of cover story for the police to explain things away.
I think the key to everything is Daniel saying 'I cannot imagine my mother killing my father, but I can imagine him killing himself'. He did lie, but not because he thought she had killed him, but because he was afraid that she might be found guilty anyway because of a lack of clear evidence.
The trial process in France looks awful, utterly brutal.