What I am saying is that it is best to leave the rebooking to the airline, even if they don't do it automatically. In the half dozen cases I faced something like this, it was either changing the flight with options in the app I had, or calling the airline to arrange something. Too bad that Air Mauritius did not want to rebook you on a partner but booking an alternative yourself is simply risky.

I am not sufficiently familiar with houw French oversees territory is handled in EU 261, but with luck NickB is right and then you could use that. If Air Mauritius is not responding, file timely with the French authority (once had a cancellation of AF from GVA, filed EU261 with them to no avail, by the time I contacted the Swiss authority a year had elapsed and they said they would take notice but not do any follow up for incidents > 12 months old)