Update: We took the train from the Paris Gare Saint Lazare train station to Bayeux, where we stayed near the cathedral. We took a taxi from the train station to our BnB, because it was drizzling, we had a lot of luggage and one of the travelers with me has difficulty walking. But many other travellers were walking with their luggage into town. Within Bayeux, it is easy to walk around without a car. We took a guided tour of US D Day sights and beaches. We considered renting a car for MSM but ended up taking the Churchill Shuttle to and from Mont Saint Michel.
I am glad that we took the MSM shuttle, at least this first time, because although the drive to MSM looks easy on the map, there were several narrow roads with minimal visibility and hidden intersections. I had read a lot about driving in France, and the "priorite a droit" plus the hidden intersections made me glad that we took the shuttle to MSM.
The large autoroute on the way to MSM looked just like any modern highway but the country roads were different. Unlike the US, the roads in the French countryside had few if any signs to let you know what road you are on. These country roads were often very narrow, without any shoulders and with lots of curves and other surprises. Families seemed to live in the same towns for hundreds of years, generation after generation, and don't need signs. Our van driver seemed to know where to go just by landmarks. My cell phone/ wireless coverage in rural France was very patchy and weak, so I would not want to be obligated to rely on my GPS. My travel companions all said that they had "no sense of direction," so that also added into the equation.
Good luck, Bon Courage whatever you chose.