What about Air Mauritius?
You want to fly them ? Better not !
I have flown 20-year old DC10 from VIASA and their were in better conditions than the A340 from MK
, they have PTV, but not on demand. The service and the meals are horrible. And at the moment the also have huge problems with their on time performance.
The airport in MRU
is small but nice. There are some streets around the runway and the taxiways so I guess there are some good sites for spotting. I haven't seen a terrace at the building itself.
I think most of the MK
flights leave in the morning for their Europe-flights (if they are not delayed). This might be the best time to see some movement. Otherwise there is not a lot of traffic, some ATR, some 737 from Air Austral.
Don't go there in Summer because you might have a lot of rain ! - although Hotels are cheaper.