MSM?

Let's start by working backward from the time that you need to be at CDG for your flight to the US: that is 11 AM. The first train (7:42 AM) or bus of the day on Sundays from Caen to Paris arrives in Paris after 10 AM for the train at around noon for the bus and that is not at CDG. I'd rethink this idea of spending two nights on Le Mont Saint Michel and spend one night and head to Paris on Saturday. You really don't need two nights in MSM to see it at night -- one night is enough. As to staying in Bayeux, it is almost a two-hour drive to MSM so I don't think that it is better if you want to see MSM at night after the tour buses have left. What time of year are you traveling?

WOW! thank you so much for this. We will be doing this in 2 weeks. Our flight leaves in the 25th.
How would you suggest the best plan for where to stay the one night and see it properly?

I made some edits to my initial response to make it a bit more clear about what time the first train leaves Caen and arrives in Paris. My overall advice does not change. What there is to see at night is the illuminated abbey and the peace and quiet after all the busloads of people have left. I suggest staying on the actual island as opposed to the hotels on the mainland. Of course, the island hotels are more expensive. I went 5 years ago in 2011 (oh my time flies) and I have heard that since the change in 2012 to the road accessing the island staying on the island is a bit of a pain because of where you have to park and transporting your luggage but I don't know if that is true. I can't recommend the place where we stayed, La Vielle Auberge. It was not horrible but it was not great but I have a feeling that all the hotels on the island itself are going to have the same issues.

What town or area would you suggest if not staying on the island?
Can we get on and off the island at any time?...or does something close?
What about the mudflats? Do you recommend doing that?

Thanks again!

Do you have to return the car in Caen? It's completely out of the way. Rennes, the capital of Brittany, is much closer to Mont St. Michel and has several TGVs a day to Paris, even directly to CDG.
You should spend the night before your flight either at an airport hotel or a Paris hotel, not on the Mont Saint Michel or Bayeux. The stakes are too high if you have to hurry through check in or miss your flight, unless you are ready to buy new tickets or pay a penalty.

We were on the RS Heart of France tour in 2013 and stayed one night on MSM. We had the luxury of leaving most of our stuff on the bus, taking only an overnight bag to MSM. There are no cars on MSM. We were dropped off by the shuttle bus just outside the island and had to maneuver up narrow cobblestone streets that all went up. Our room had a magnificent view. Many many stairs so pack accordingly.

I would definitely stay on the island. There are not a ton of hotels there, so just look on booking.com and see what is available. To have your own room there is the best way to experience the magical ambiance of the island. Yes, there is also a beautiful view from the mainland, but there is nothing stopping you from walking out a bit to see the lights. If you wake up in the middle of the night and look out the window it is pretty amazing. I went in Nov and it was truly one of the most memorable experiences. If you don't stay you can still enjoy it, but I have never heard anyone say they did stay and regretted it.

It is one of those experiences that makes me long for Europe. I took my 16 year old grandson and it is a very special memory for him. It's just not something that everyone does, so that is part of what makes it special.

Preferences are going to differ as to whether you "should" stay on MSM or not. I stayed at one of the string of hotels just across the bridge last fall and thought it was fine. They have frequent trams across the bridge that stop right by the hotels so if you want to visit it at night there is no problem doing so. I would not recommend 2 nites at MSM, I think most people find 1 nite is plenty. Do what RS says in his book, arrive in the late afternoon when the crowds are thinning. Note that restaurants in the area (both on MSM and across the bridge) close early so plan on being seated by 7pm. Then visit again at nite, and the next morning too if you'd like.

4 nites in St. Malo wouldn't be my first choice. As one of the RS staffers posted in https://community.ricksteves.com/travel-forum/france/sunday-in-st-malo-e927d68a-e781-4243-8029-9a16edd9b35f the main attraction of St Malo is walking around the ramparts. You can do that, visit the shops, have a meal, etc in a few hours. After that St. Malo is just your base and parking is going to be limited and somewhat expensive (unless your hotel offers it). I'd instead pick a town such as Dinan as a base and visit St Malo & Dinard across the river as a day trip.

The info on the region is skimpy in the RS book. I recommend the Rough Guide for more detail, and the Green Michelin guide will have info on just about any town worth visiting as well as some suggestions for driving tours.

Also think carefully thru your plan to get back to Paris. The way I read your 1st post you are going to drive to Caen, return the car, and get on the train to Paris. The rental return place may not be open Sunday, or may open later than you need to be on the train.

About Saint Malo, just to add some information, some parking areas are free, it depends where your hotel is located. Around the intra muros area, yes, you have to pay. But along the Sillon (where are some hotels) or on parallel streets you can find some places to park.
There, if you intend to take your car and visit not only Saint Malo, there is quite much to see around : Dinard, Dinan, Cancale, Dol, Cap Fréhel / Fort la Latte, Fougères, Rennes, Combourg...

I'm hoping to stay in St Malo. I just finished the novel All the Light You Cannot See (St Malo is one setting) and my husband is a commercial fisherman in the states. It feels like we would like it there.

I changed from staying 2 nights in MSM to just 1 ...and we will turn the car in on Sat. In Caen....and head to the airport area to be ready to fly out Sunday afternoon.

Because it's on a peninsula, it takes 15 minutes to get to the main highway from St Malo to begin a daytrip, not counting traffic and bridge openings. Parking is quite an annoyance there too. We also hate to change hotels, however. If the hotel promises hassle free parking, then indulge your interest in St Malo.

Depending on your interests and desires, would you consider returning the car at CDG and staying in an airport hotel? Then you could visit Giverny, Rouen, or Honfleur on your last day?

I get the feeling that you have not done reading on your destinations ... No one ever proposes two nights on MSM. There are lots of other sights in Brittany, and you might make better use of your time.

The order of your trip is wonky. Are you literally going from St. Malo, to Bayeux, to MSM and then back to Caen?

Can you go St Malo to MSM to Bayeux to Caen? This will save you a tremendous amount of backtracking.

Train from Caen to Paris is only a couple of hours, so if you go back on Saturday, you'll have time for a leisurely breakfast and last stroll through Paris before a 2pm flight on Sunday.