I actually disagree with the advice to give up 2 days to travel such a short distance.
If you have to be on the ship by 4 (pm I presume?) I'd plan to be there by 3. If Nice or tendered from Villefranche sur Mer, half an hour by taxi from either the Nice airport or Nice Ville train station should be fine. Where is the boarding place?
The 9th of May this year is a Thursday.
The plane from Paris takes less than an hour (after the pre-boarding security theatre). I am assuming (correct or no?) that when you say you will have been in "France" since the 5th you mean Paris?
Even if there is a problem with a plane on a very early flight (unlikely) there are other planes on that route aren't there?
Personally I would take the train. No security theatre and very easy to do, and perhaps cheaper if you have cruise ship luggage.
The 6:07 TGV from Paris Gare de Lyon arrives in Marseille-St-Charles at 9:39, an easy 19 minute connection to a fast local train at 9:58 (grab a take-away coffee as you wait) which only calls at the 6 major stations on the way and deposits you at Nice Ville at 12:38. Plenty of time for a leisurely lunch at the Paul immediately in front of the station. Then get to the boat.
If you want to cut it a little finer, there is my old friend the 9:07 which is express to Marseille-St-Charles and then calls at the same stops as the local train, arriving at Nice Ville at 15:06, no changes.
The demonstrations have been generally on Saturdays, and there is a long time between now and May for it to diminish further.
Strikes are advertised well in advance and are unlikely.
We all have different degrees of worry and stress.
If don't mind leaving Paris a day earlier, then you could do a whole lot worse than a day on the French Riviera. Can you manage Paris with only 2 and a half days there? Have you been to Paris before? Have you been to the Riviera before?