Thanks so much to this forum for the earlier posts related to travel to MSM and St Malo without a car. We now find that it is doable to spend a night at Mont St-Michel and then travel on directly to St. Malo. Currently on the schedule is a direct, early train from Paris Montparnasse station arriving MSM around 12:30pm. With a one-night stay, this allows an afternoon, evening and morning to view the island and tour the abbey, explore, etc. Day two we had planned to train to Dinan. It was complicated. From this group, I learned I should search for a shuttle bus. An early afternoon daily shuttle (cheap!) from MSM and St Malo is offered by Flixbus. (Also offers St Malo to MSM.) We'll spend a couple of nights in St Malo and then take the train to Vannes (hopefully with a stop in Dinan for the Thursday market.) After Vannes, a return train to Paris via Rennes.
By the way, there are also direct shuttle services between Bayeux and MSM. But the available transportation services all seem to be round trip on the same day and cost a bit more (~100 euros) - Viator, Get Your Guide, Churchill Hotel in Bayeux.
Maybe this will information will be useful to others!