MSM Mont Saint Michael from Paris in the evening

Your plan is good. But a better plan would be to not leave at 10 pm, but instead spend the night in one of the few little hotels on MSM itself. Then get up early, enjoy a few more hours with the place all to yourself, then flee as the crowds pour in around mid-morning. Book your hotel far, far in advance and be prepared to spend a bit more than you ordinarily would for basic accommodations - it's all about Location Location Location.

Normandy and Brittany (MSM is right on the border) are both really beautiful. Even on a busy, short trip, it's absolutely worth giving them another day or two if you can.

I don't think there's enough to do on the Mont, especially if youndon't have a car. Sure, it's picturesque, but the Mont itself is isolated. Well, if you don't count the Alligator Farm and tourist traps on the highway in!

I'm not trying to talk you out of it. It's a must do (if not on your first trip to Paris, necessarily.) But put it in perspective-there are dozens of lovely medieval towns in Europe. This is one of them. To me, it wasn't worth 24 hours. Opinion.

For an alternative to visit Mont Saint Michel from Paris and to avoid crowds, you may take a train to Rennes or Saint Malo, and then book a private tour to visit not only Mont Saint Michel but also St Malo (or Rennes), it allows you not to lose time looking for interesting places.
As others said, it's a pity to drive that far without seeing at least a few other places in Brittany or Normandy. Thus you could enjoy better conditions to visit in the late afternoon/evening.
But if you really have only 1 day available, it's still possible over the day from Paris (and take the last train from Rennes to Paris; depending on the day, the last train can be at 9pm or 10pm). You can have a look at local tours on http://visite.bretagne.free.fr/index.php/en/nature/mont-saint-michel.html