Cryptic "spoilers" ahead.
The key scene in the movie is the morning after with the magazine reporter and his old girlfriend, because his attempt to go back in time is the thematic counterpart to the time travel attempt.
There is no way for Mark Duplass' character to rig a crash, which means Belinda never died.
There is an ambiguity between just disappearing and going somewhere.
The movie is bittersweet.