Travel from Dijon to Mont St. Michel

I can't answer #2, but you seem to have a good practical plan, although it odesn't seem like you will have much time at MSM. If you are staying ion the Mont, that will work. Also, if you are planning to spend some time in Le Mans (I hear it is a nice town to visit) you will be really cutting yourself close in Chartres. Have you checked train times? Are there late enough trains from Chartres to Paris to allow you enough time in Chartres? Day 2 does seem to be crunched up.
I wish you luck!

Buy your Dijon-Rennes ticket as a single transaction from SNCF: that way, if the Dijon train is delayed, you are covered.

60 minutes would be a good amount of time for the transfer. Avoid the bus, it is unpredictable; best to take metro line 14 & 6, or just walk 7-10 min to line 6 at Bercy (exit Gare de Lyon via Hall 3, rue de Bercy side, taking a left as you exit).

I don’t know what the rest of your trip is but this is a lot of time and logistics to just see msm and Chartres. Save it for next time or reconfigure your trip to focus on the msm area. There’s plenty to do in both regions. Having done the drives between burgundy and Brittany, Brittany and Chartres, and Chartres and Orly airport I can say that while long and boring the drives are not challenging. I would choose driving over what you’ve proposed, especially if there’s a few of you.

Thank you, Judy, Balso and Matt. We have been to France many times but have not done the long drive from Dijon to MSM in more than 20 years. This time, we are taking a small group (7 persons including my wife and me) on a 10-day trip and plan to visit Taize in Burgundy, MSM, Chartres and Paris. Per Matt's advice, we will consider driving instead of train. We plan to rent the cars in Dijon and return in Le Mans as that seems to be the car return location nearest to Chartres. (I need an automatic car.)

With a group of 7?
You can rent a 9-seater van, and it is a better option than the train IMO, but there is a big caveat: luggage space on these vans can be surprisingly limited. Your party should travel with carry-on luggage only, if possible.