If you fly into Nice and spend just three nights there, you'll probably only have about 2 days of sightseeing time. The arrival day is often experienced in a sleep-deprived, jetlagged haze. Your time will be very short, given all the attractions in that part of France. Traveling from Nice to Marseille would take over 2-1/2 hours one way, so I would definitely not try that as a day-trip. There are many lovely small towns and villages much closer to Nice. Pick up Rick's guide book to southern France and/or another book that covers that area comprehensively. See what place(s) sound most appealing.
Trips on the local trains are inexpensive, and the tickets needn't be booked in advance to get a good price, so you can head off to Monaco one morning without a fixed idea of how much time you want to spend there, then perhaps visit another coastal town that same day. If you happen to be art lovers, you'll probably have a difficult time dragging yourselves away from Nice, which has at least six good art museums.
I agree with the concern about trying to squeeze in an additional stop on the day you travel from Mont-St-Michel back to Paris. Rennes is an attractive regional city, but the train schedule doesn't seem to work well for a stopover. You'd have to get to Pontorson in time to catch the 8:42 AM train, then find a place to stash your luggage in Rennes and allow time to retrieve it before continuing your train journey less than two hours later. And if I'm not misremembering, the train station isn't particularly near the historic district. This plan would make me nervous about missing the onward train to Paris.
The fact is that there is very limited train service to and from Mont-St-Michel, which makes for a certain lack of flexibility when traveling there by rail.
You are not allowing much time in Paris itself. On Day 4 the trip from Nice to Paris will take at least 6 hours just on the train. By the time you deal with hotel check-out, getting to the station, finding your way to your Paris hotel and checking in, that day is going to be pretty well shot. You'd probably save a bit of time by flying, but you might reasonably judge that it wouldn't be worth the hassles involved in dealing with airports.
However you get to Paris, you'll basically just have Days 5, 6, and 7 there. On Day 8 you head to Mont-St-Michel. Unless you want to take the 7:40 AM train (which gets to Pontorson at 10:46 AM), you'll spend 4-1/2 hours getting to Pontorson at 2:40 PM, then take a shuttle bus to Mont-St-Michel. On Day 9 you'll return to Paris at 3:07 PM or 6:04 PM, with the day fairly well shot from the sightseeing standpoint by the time you get checked into your hotel. And Day 10, I assume, is the day you fly home, so really only 3 full days in Paris.
Are you sure you want to spend 2 days traveling to and from Mont-St-Michel when you'll have so little time on the Riviera and in Paris?