Rhoda-2 won the essay contest, not by writing about her criminal experience which she never had, but by becoming amazingly successful.Rhoda-2 never killed John's family and was never charged with drunk driving. She went to MIT, got a fantastic career as whatever, and used that experience to win the Essay Contest-2, while Rhoda-1 used her experience as a felon. Since Rhoda-2 never met John, she went to Earth 1 by herself.
Earth 2 is a doppleganger world.The moment after the film ends, the Earth 2 double of Rhoda mutates into something ugly and devours Earth 1 Rhoda. It turns out that Earth 2 mimics planets and their populations to feed on them. This is why it took a while to establish contact, and why the Earth 2 SETI woman's voice was choppy - the planet was still manufacturing doubles en masse as it scanned earth at higher and higher resolution. Earth is totally screwed.
The final scene when Rhoda sees her double is actually on Earth 2Either one could be the Earth 1 Rhoda.
The character of Rhoda isn't actually as attractive as the actress playing her.We can't call Hollywood Homely because it's never stated, but a woman of 22 years old and being that pretty should have had guys asking her out and trying to include her in their social lives. So maybe she really wasn't very pretty.
Rhoda was looking up and right instead of left on Earth-2Thus, she swerved right instead of left, and didn't hit anyone.
The Earths are moving towards a collisionAside from the obvious scientific likelihood of that happening, every time Earth-2 is show, it is shown bigger.
Earth 2 Rhoda killed only John in her version of the collision.Both Rhodas probably looked up, and crashed into the families car, but that's when events slightly changed. So how Earth 2 Rhoda might have crashed into the car might've been a way where only John (or at least John) died. The main reason why I don't think it possible that the crash never happened on Earth 2, is that she needed to have that reason to write down her entry to win a trip to the other Earth about sending outcasts and felons like herself. If she was successful in life, then there no way they choose some random college graduate looking for a good time. Therefore, if she only killed John and she would've still written that entry but not had any romantic relations with the family left behind in her world and think about giving up her spot. So in the end we see Earth 2 Rhoda coming to Earth 1, as a chance for escape or a new beginning.