As a born Mauritian, I would say that it really depends on whether you are happy eating outside the hotel or not.
I stay at hotels when I go back on holiday but only tend to eat dinner in hotels because they tend to do buffets in the evenings and I have never been disappointed.
A meal in a hotel will be around £20-£40 and an alcoholic drink under £10 i.e. whilst expensive, no more than say eating and drinking when out in London.
Eating outside the hotel is so much cheaper. Eating fast food or small reastaurants, meals will be under £10 a head and streetfood £1/2. Even nice restaurants, you would struggle to exceed £20 for a very good meal.
So in summary, all inclusive imo isn't worth it if you are happy eating out. Compromise would be to have dinner at hotel but eat out for lunch (half board).
Le Morne is gorgeous and probably near the most beautiful and scenic part of the island but as said, the North/North West is a bit more lively and has calmer seas for swimming. Beaches are similar down South but not really great for sea swimming.