Getting to Mount Saint Michel from Bayeux or Caen

Yes, we took the train from Bayeux and then the connecting bus a few years ago. It worked out well. But we only travel by train, not car.

Renting a car likely would be more expensive but would save you a bit of time. It also gives you flexibility -- there are a lot of sights in the area you won't see from a train.

Between Bayeux and Saint-Lô is the réserve naturelle nationale de la forêt domaniale de Cerisy, which could be quite pretty in the autumn, depending on when in October you're traveling. also in the forest are the 17th century Château de Balleroy and the 7th century Abbaye Saint-Vigor and its gardens.

The château has nice gardens and, of all things, a museum and small cinema room (in French and English) focused on the history of hot air balloons (in French: montgolfiers, named after the Montgolfier brothers who invented and designed the hot air balloon familiar today). Oddly enough, the château has been under ownership by Americans since the 1970s, initially by Malcolm Forbes, who had an interest in hot air balloons and created the museum.

One problem with traveling on Monday is that many restaurants are closed that day. Of course, there's always that internationally known restaurant with golden arches and they're open Mondays: they have outlets in Bayeux, Saint-Lô, Avranches, and just outside Pontorson.

Thank you for your guidance. I forgot to add that we are staying overnight on Mount Saint Michel and leaving the next day.

Another vote for a car. It isn't that far of a drive, and there are some nice things to see en route. Avranches is a nice little town to stop for a meal or sights, and there are several towns of note along the way with significant WWII history.. The drive itself coming from the East on the D43 offers excellent views of MSM.

We took the train from Paris to Rennes; picked up a rental car and drove to Pontorson where we stayed and then visited Mont St Michel. After that we drove to Bayeux for a 4 night's stay. Driving in the French countryside was so easy and I personally was glad we had a car. Though you definitely want to make sure if you have the car in Bayeux that your accommodations include parking.

Avranches is a nice little town to stop for a meal or sights

If you do stop in Avranches, make it a lunch stop and eat at Le Littré, if possible. It was fantastically good value!
Since you're staying overnight on the Mont, you don't need to arrive until after lunch anyway, and going the other way, you can definitely leave before lunch: there's not that much to do on the Mont and the food is decidedly underwhelming, so the fewer meals you eat there, the better!

Debbie, If you decide to rent a car and use GPS or Google maps or whatever for route and driving directions, please note the spelling you input. It can sometimes make a difference in France. It is Mont St. Michel, no "u".
Enjoy your visit there. It is a one-of-a-kind place!