Yes, that works. An alternative is the TGV high-speed train.
Teams appear to be near LeMans, so that has the best train, all the way to CDG 1133 1311.
Nantes is another possibility. It has TGV service to Paris-Montparnasse 0730 0941 or 0900 1115. From there the Air France shuttle bus takes 30 minutes.
Teams have to get to Rennes about 37 miles from Mont St. Michel. If they reach Rennes before 8am, they can take the 0803 train arriving Paris-Montparnasse at 1022. Then they can get a 30 minute Air France Shuttle bus to CDG.