Transportation from Paris to Rouen and return to CDG

Rail is an option. Direct trains Paris St Lazare to Rouen, trains Rouen to Bayeux with a change at Caen, trains Rouen to CDG with changes at St Lazare and Gare du Nord. Get an early start with the latter as you must be at CDG before 10 am, and 9 am is better still.

There's rail service from Paris's St-Lazare station to Rouen, taking as little as 76 minutes. That doesn't cover the time to get from CDG downtown, and of course you'd need to allow an unknown amount of time in case your flight is late or you encounter delays in the Immigration process. The train fare might be as low as 24 euros per person; I don't know whether any of your travelers are of an age that might allow a discount. I have no experience arranging private transportation for groups like yours, so I have no idea how much more expensive than the train that would be. You would of course pay for the round-trip journey even though you'd only be traveling one way.

Most of the D-Day tours originate in Bayeux. They use vans, not big buses. At least some of the companies do private tours as well as public ones, so I think you might be able to arrange a private tour for your group. You've left it kind of late for a tour at the end of December, but I would still try. The company that conducted my public tour was Overlord, and it's one of the largest companies, so I'd suggest starting there unless someone else comes along who's had experience with a private tour for a group like yours. I'd think for a private tour you could probably arrange to be picked up in Rouen and returned there--at least I hope so, because it would avoid the need to wrestle your crew onto trains, and it appears the first train from Rouen may not reach Bayeux until 12:26 PM.

I would not be comfortable spending my last night in Rouen before a 1 PM flight back to the US. You should be at CDG no later than 10 AM, and the trip back from Rouen will probably take somewhere between just under 3 hours and 4-1/2 hours. The trip involves a change of train stations; trains to CDG depart from Gare du Nord rather than St-Lazare. Although trains are not affected by highway traffic, occasionally something does go wrong. One of my trains in France was cancelled unexpectedly back in 2017. I wouldn't take risks with a transatlantic flight hanging in the balance. The cost of a last-minute, one-way replacement ticket home would probably be very high indeed.

Normandy's invasion beach visits typically use Bayeux as a base. Ticket costs start at 15€ (or more, depending upon when you purchase tickets) and depart from St Lazare in Paris.

Train tickets to Rouen start at 9€ each way. For more information:

https://www.sncf-connect.com/

From Rouen, you can take the train to Bayeux from where you can easily visit museums and the beaches themselves. Tickets from Rouen to Bayeux start around 30€ per person.

I'd spend the last night at the airport; we use the Novotel which is right on the RERB and the CDGval tram that goes to the terminals. They have a good breakfast buffet as part of the room cost. Take the train back to St. Lazare that evening and then get a couple of cabs to the airport for 53 for one and 57 for the other for your party of 9.

Eat dinner in Rouen and arrive at CDG late in the evening and sleep there which makes it easy to be in line for check in at 10 am the next morning. While trains are reliable they are not perfect. We were once on a Thalys from Amsterdam that came in 5 hours late -- lots of people missed CDG flights. Make that last night easy.

Rouen is a wonderful little medieval city, and it is in Normandy. But it isn't at all convenient to the D-day beaches. Bayeux, as others have mentioned, is where a lot of tours originate and is also a wonderful medieval city. There is also one of the best D-day musuems in Bayeux (nothing like that in Rouen). Unless you have another reason to base yourselves out of Rouen, I would reconsider. Either city is easy to get to from the Gare Saint-Lazare train station. Like others, I wouldn't stay in Normandy the night before a flight out of CDG. Too much time, and too much could go wrong getting between the two places. Figure an hour each way to/from the Paris train station from CDG, and that doesn't count check-in, luggage, passport control, etc. I believe it is about a two-hour train from Gare Saint Lazare to Bayeux.

I agree with the others about not staying in Rouen unless you have another reason to be there but it might be too late now to change. Assuming you have some reason to stay 2.5 hours away from the Normandy D-Day beaches that you mention and almost 2 hours from the best museum (the one in Caen), there is train and bus service between Rouen and Caen. The trip will take roughly 2 hours. I know that there is at least one tour of the D-Day beaches and the Caen Museum that departs from the train station. A friend of mine who was in Caen for non-vacation reasons took such a tour. The one that he did is not operating in December and was limited to 7 people but I am sure that a Google search of "tours Normandy beaches Caen" will yield results.
Like other responders, I would return to CDG the night before. If I was traveling alone or if my flight left at 4 PM or later, I might head to CDG on departure day but for a group of nine, I think it is much better to be at CDG (or maybe just be in Paris) the night before. I'd eat dinner in Paris because the last train from Rouen leaves at around 9 PM.