Sept. trip 15 days; Paris In/Out. A couple of days on front and back end, with maybe Giverny, then Normandy area WWII sites, etc., MSM, and Brittany area. Train to Caen?, then car rental, drop off in Rennes? back to Paris vis train. Do I get the car day 1 or do a WWII full day tour and then rent for the duration? Approx 8 days for Normandy and .Brittany.
I always recommend at least a one-day invasion tour for anyone who isn't a real expert on the subject. I think Bayeux is a more pleasant place to stay for a visitor to that area who will have a car, and that's the point of departure for most of the tours. In addition to the (smallish) historic center, cathedral, tapestry and walking tour offered by the tourist office, Bayeux has a good invasion museum. The upshot of all this is that one could fill about an extra day in Bayeux.
Caen has more car-rental choices than Bayeux. If you rent from Bayeux, I don't even know that you'd be able to return the car in Rennes; you should check on that. You might want to head all the way to Bayeux initially, then hop on the train back to Caen on a subsequent day to pick up the car there when you're ready to use it. But do check agency hours of operation very carefully. It has been reported that plenty of places close for lunch, and weekend hours can be limited.
An advantage of allowing two days in the Bayeux area is that it may turn out that your invasion tour of choice isn't running every day. That's one of the first things I'd do: Contact the company and ask about that. Ideally, I'd try to take the tour on my first full day in that part of Normandy so you'll be in a better position to decide on other related sights to see on your own afterward.
Eight days sounds like a lot, but Normandy + Brittany is a lot of ground to cover.
Depending on your full day tour, I would suggest getting the car in Caen and drop in Rennes. For great itinerary check out Frommer's 25 Great Drives in France. They have 3 great drives around Normandy and Brittany. We have been to that area 4 times and love that part of France.