Mont Saint Michel Luggage issues

I've witnessed young, healthy people struggle with luggage on MSM. Particularly those things with four wheels that roll smoothly in airports—but not so much on cobblestones. I'd plan on carrying whatever you must on your backs. Worst case, 2-wheel roller-boards. Trying to schlep huge suitcases on the Mont will be a painful, humiliating experience IMHO.

I don't remember the inside or the outside as having cobble stones. Shouldn't be too bad if you are staying in a hotel on the island. The worse part is the crowd when you not arriving before 10 or after 4

The main street (which you will undoubtedly use to get to your hotel) is cobblestone. It is worn fairly smooth and looks to be thinly paved in some spots. But you will find it difficult to pull rolling luggage up the street.
I don't know of any options. Perhaps you could contact your hotel and ask if you could pay to have someone assist you.

Are you returning through Rennes too? If so, there are private online services that offer baggage storage near the station (there is no storage in the station). You could try Nannybag or Eelway, but there are probably others. Usually they contract with a nearby hotel, so there is 24/7 service. I'd avoid shops that are just open business hours.

There is an hostel in Dol de Bretagne, they do luggage storage for the day. Right behind the station. Direct bus to Mont Saint Michel every morning (30mins drive)