IMO, your best bet is to return the car in Rouen, then train from there into Paris. There are rental company offices right next to the train station (I'm renting from Hertz there but there are others too).
Make sure you pick up an International Driver's Permit at your nearest AAA club before your trip. They cost $20 (plus $12 passport photos if you don't have an extra). Each driver is required to carry their driver's license from home and an IDP (often mistakenly called an international driver's license - there's no such thing as the license).
Cars on the right have right of way. If you're on a main road and a car, or bike, approaches from a small road on the right, they have right of way. This isn't true in a round about, the car in the round about has right of way.
http://www.francetravelplanner.com/trans/car/priority.html