Need advice for last-minute Paris itinerary and commuting in from Marriott Village d'Ile-de-France

I have driven in Rome and Morocco and Paris so I think you can certainly handle Paris driving. To me what makes Paris driving difficult is congestion so I suggest driving to a stop on the metro system in an inner ring suburb and taking the train in from there on the days that you want to be in Paris. When I went to the opera in Paris I was sitting next to a woman from Vincennes who said she used to drive into Paris for the opera but she hates driving there now because of the congestion, which she blamed on the new bike lanes. If you are going to drive into Paris, make sure you have a Crit'Air sticker on the rental. I assume that the rental car will have one but make sure.

It is 47 minutes from Disneyland to Gare de Lyon on the RER A. By the time you drive to a suburban Metro station, park the car (not free) and get on a Metro train the rest of the way into Paris, you could have been there already. Driving into Paris on a weekday morning would be quite a headache. Tourists have very little impact on Paris traffic congestion. People going to work (it is the political and financial capital of France) will be the challenge.

Edit, there is very frequent bus service (No 34) from Marriott Village d'Ile de France to Val d'Europe RER A station. For 22.80 EUR each, you can all get Navigo weekly passes which will cover all public transport into Paris and all over Paris and back. Starts midnight Sunday night until midnight the following Sunday.

What Sam said. You knew there would be tradeoffs in taking accommodations so far out. Renting a car for trips into Paris will likely cost more in money, time, and aggravation than just using the RER and Metro.

Normandy in late November would not be my idea of an enjoyable time, unless you had a stretch of unseasonably warm and sunny weather. I've been to France a couple of times in November, but for me, the cities have more to offer than the countryside at that time of year.

I wouldn’t go to Normandy in late November. We were there over Thanksgiving in 2019. Almost everything was closed in many of the small villages. The DDay sites were open, but no restaurants and most shops close. One day we had a picnic in our car of croissants and quiche from a bakery because every restaurant was closed. Even in Bayeux at least half the town was closed until 12/1.

On that same trip we did a nighttime bus tour through Paris to see the Christmas lights. The congestion is crazy. You will spend a great deal of time going nowhere if you plan to drive in the city. I would imagine that a taxi to/from the hotel to the train station would be less expensive than gas and parking in the city. And much less stressful.

The schedule for Bus 34 can be found at -orme-rond-vers-gare-de-marne-la-vallee-chessy-nord/510-510-695622825-695622763 if you decide to go that route. I still like your idea of renting a car.

Traffic is a major issue, but I'd still rent a car.. If only to go to Disneyland Paris RER station. Or you could perhaps park further down the RER line, such as Noisy-Champs (traffic gets really bad west of Noisy-le-Grand).
With the car, you have the freedom to explore places outside Paris: Provins, Senlis, Chantilly, Fontainebleau, Vaux le Vicomte, Moret sur Loing... are all reasonably easy to get to from where you'll stay.

1) since this is low tourist season, will the driving/parking in Paris be that bad?

Tourist season doesn’t have much, if anything, to do with traffic in Paris, which is an absolute nightmare thanks to Mayor Hidalgo’s various initiatives.

Take public transportation. It will take the same time in a car by the time you are stuck in traffic, and your frustration level will be off the charts if you are stuck in that congestion. Mine would be, and I know my way around .

This is simply the cost of the bargain lodging opportunity you have.

is there another line heading to the 18ieme? Where will these new lines go and will they go into the centre, or are they all peripheral?

All the new lines are either extensions of existing radial lines, or peripheral lines - nothing within Paris city limits, as there is no need.
In any case, traffic around Paris has been bad for many decades and, to be more relevant to the OP's question, the RER A from Marne La Vallée / Disneyland into Paris is probably the best commuter line - fast, frequent, and reliable... with crowds to match at rush hour.

Or from Val D'Europe, one stop away from Marne La Vallee and perhaps a little closer to the Marriott with a bus connection. Since these stations are at the beginning of the line, even when they eventually get crowded as they approach Paris, they will be empty with seats available at the start.

It is nice to get a seat on the RER-A. I wonder if the RER-A still has cardboard seats?

Strangely enough, I was literally reading Nigel’s question about the RER A and cardboard seats just now as I was stepping on to an RER A train (not something I do that often, although the branch that goes out to Disney has its last Paris stop quite near my house).

Alas, standing room only, so I have no idea what the seats are made of !

P.s. not all of us Parisians voted for Mme Hidalgo and her faux green credentials in the mayoral election last year, just sayin.

We stayed at the Marriott for a week 3 years ago. (We tried to trade to a Marriott in Hilton Head with no luck, so we decided to try Disneyland Paris) The resort is really lovely and we had a fabulous detached house (2 bedrooms, 2.5 baths and a lovely patio) We rented a car for the week we were there and bought Navigo passes as well. Since we had been to Paris before we explored some of the area around the Marriott.. Our itinerary was:
2 days in Paris
Versailles
Provins
Reims
World War I museum/Meaux

If we had another day I would have opted for Chartres, but that will have to wait for the next trip! If we had not been to Paris previously, we would hve gone in at least 1 more day. With the rental car we drove to Val d"Europe to catch the RER train into Paris. (Marriott to center of Paris was about 1 hr 15 minutes including parking.) The car also made it very easy to go to a variety of restaurants for dinner on the days we didn't go into Paris. There is a small shopping area in Bailly Romanvilliers with grocery store, bakery, etc. but I must say the market at the Marriott was very well stocked. In retrospect, it was a great week!

Wow, Rick Steves followers are the best! Thank you all so much for the thoughtful advice. We have finalized our plans to stay in the city center for the days not at the Marriott near Disney and forego trying to travel to Normandy or other farther areas. We are renting a car, but only to explore areas outside of Paris including some of the spots you all recommended. Now, wish me luck that either we are able to get the passe sanitaire from a pharmacy once we're there or that our CDC cards suffice to enter all the sites. Thanks again!