Last 2 weeks

That looks like a good plan for your last two weeks, maybe a little rushed for my taste but you seem to move faster than I would anyway. That late in the season you could probably find accommodations without too much advance planning -- you're driving, right?

Do some research now on potential places to stay, look at availability, maybe nail down your Carcassonne accommodations and have candidates for your stops in Sarlat, Amboise, and beyond. (I'll just put in a plug for Le Petit Versailles in Beynac, outside of Sarlat.) As you progress through those two weeks, be ready mentally to change plans, maybe stop at Albi on the way north from Carcassonne, or stay a little longer in the Dordogne or Loire if you're enjoying yourselves and want to see more caves or chateaux or whatever. Line up your reservations a few days ahead instead of trying to put it all together now. You might not make it to Brittany or Normandy, but they'll be there for your next trip. If at any point you decide you've had enough of the countryside, head back to Paris for your final days or week or whatever. There will be more than enough to see and do, even after your previous five days.

Plan to spend the last night before your flight at the airport if it's in the morning, or in central Paris if it's in the afternoon, having dropped the car on the outskirts. You don't want to be dashing down the highway to CDG on your flight day and run into heavy traffic or delays finding the rental return or whatever.

I think your "spur of the moment" focus is a good one, especially in the off season. You'll never see everything worthwhile in the time you have, no matter how carefully you plan in advance. "Assume you will return" is the best advice.

Normandy is not optional. The Dordogne is also a big draw. So given our time constraints...and yes...we will have a car...and we will have already done Provence...what would you suggest for how long and where to spend it?

Thanks again!
Kim

So actually you have two weeks to drive from Marseilles to Normandy, visit Normandy, and get back to CDG. If Normandy isn't optional, then you might as well make some plans for that part of your visit, working back in time from your flight day. I'd suggest basing yourselves in Bayeux, but including an overnight at MSM, perhaps before Bayeux. We stayed at the Novotel on Bayeux outskirts, which was about a 15-minute walk from the center and convenient for driving elsewhere, but otherwise just a typical chain place. If you post a question about Bayeux (or elsewhere in Normandy) accommodations you'll get good recommendations -- or just use the search feature above for previous threads on this topic.

It's a few hours' drive from Bayeux to CDG if you want to do that the day before your flight and stay at the airport. Or you could drop the car in Caen and take the train into Paris for your last night. Depends on what time of day your flight is

You can line up your Normandy accommodations based on how many days and nights you want to spend there before flying home, and leave an open period (10 days?) to get there via the areas you mentioned. Then wing it within that time window. Maybe you'll want to spend more time, or less, in the Dordogne or Loire. You should be OK with hotels as long as you're flexible and not too fussy.