I would be deeply concerned if Hainan's incentives for operating ex-MAN were directly linked to a clause requiring the continuation of services ex-STN. A new long-haul service is a high risk undertaking at any airport including MAN and failure is always a possibility (see: FlyNas). However, STN in particular has a dismal record in sustaining long-haul schedules. This is largely due to the airport's proximity to the competing offerings ex-LHR (and to a lesser extent LGW). In STN's case the LHR factor has not gone away which would make a Hainan service there particularly high risk IMO. And that begs the question: if Hainan Airlines eventually chose to withdraw from STN, would they forfeit a linked package of incentives supporting their operation ex-MAN? I sincerely hope that this conundrum is never allowed to arise.
If MAG can attract Hainan Airlines to STN on its own merits, fair play to them. But please keep the terms of the MAN arrangements non-dependent on the airline sustaining its presence at STN.