Question: If Sandra is at the scene of the accident, what is the purpose of the police officer showing up at the door to tell her Jim is dead?
Answer: Possibly she left the scene before the police arrived, so they did not know she had been there.
Question: I'ma pretty smart person but I got so lost watching this film. Is she clairvoyant?
Answer: It's supernatural, but I don't know what to call it. Linda's consciousness is moving in time to previous days, somewhat randomly, after she sleeps. When she wakes on a day in the past, she has knowledge of the future, and her actions apparently 'stick' and change what happens in the days after. She has to figure out the rules of time travel, how her husband came to die in the crash, and how to prevent it. It was a lot for the audience to follow.
Answer: It's fairly obvious that this film's theme is that knowing the future changes it. I.e. knowing there would be an accident, she travelled there and caused an accident.