I think the San Sebastian route is possibly a case of Air Nostrum trying to be a bit inventive on the use of an aircraft. It seems to fly Palma Mallorca -> San Sebastian -> Luton -> San Sebastian -> Palma Mallorca on Tuesday and Thursday
San Sebastian would normally see 6 round trips per day to/from Madrid on a Tuesday or Thursday. In peak summer this drops to just 3 round trips per day - I imagine this is due to reduced business demand.
Assuming Air Nostrum have not left it too late to sell what would be a pair of very leisure-focussed routes, this might do ok, particularly given the experience of Luton-Vigo last year and San Sebastian's large beach and other tourist-focussed amenities. Be interesting to see whether it can capture some of the people who would otherwise fly to Biarritz or Bilbao