Have you considered returning the car at the [Tours]St.-Pierre des Corps TGV station, if you're going back to Paris at the end?
Rick mentions the idea (he's talking about Rouen, I think) of getting off the train, locking your luggage in the [fully enclosed?] trunk of the car and leaving the car in the major rental company high-rise garage, and seeing a city before driving off hours later. Naturally there is risk involved, and I don't know how you can learn which rental companies have a very secure garage, and which (like in St Pierre des Corps) just have wide-open, street-level lots.
While we didn't feel that an overnight at MSM was required, many posters here swear by it. I wonder if the Hotel Churchill van service would accept your luggage and you for a one-way trip to MSM, instead of the usual round-trip. You could get a cab to a rental agency near MSM, if there is one. It's quite a touristy (descriptive, not a slam ... Crocodile Farm, for example! ) highway area.
We had a car, and went in the other direction (Tours-Chinon-Dinard(&MSM runout)-Bayeux-Paris). But I'm not flogging that route. I do want to observe that if you have reserved the time for a leisurely drive, there are plenty of attractive short visits along the way. We didn't have time for the Cider Route, but we loved Dinan, some estate public gardens, Vitré, Dol de Bretagne. I remember being disappointed with Coutances, and the time to drive there. We skipped Caen. Although I think bicycles are a great idea for the Bayeux area, I would be careful about narrow paved roads and heavy auto traffic near the D-Day sites.