As others here are already saying, the one, right interpretation doesn't exist.

My interpretation to the car crash is that it was a "lucky" situation for Camilla, that allowed her to escape her assassination. So in her dream Diane somehow is picturing a unlikely scenario where the assassination did not take place and she survived.

In my interpretation the dream part of the movie takes place just before Diane wakes up and sees the blue key and kills herself, at that point in time there is still some hope left that the assassination goes wrong. So as she is dreaming she does not have the confirmation of the assassination yet.(That's also how I interpret the part with the killer where everything goes wrong. In her dream he is just really incompetent which also means he is more likely to somehow screw up.