no, no, no - no, no, no. I don't know why you think anyone would operate from LDY - LHR, when BA don't operate to Northern Ireland from LHR and bmi go to BHD. Why would Aer Lingus operate?
There is a PSO (Public Service Obligation) service to Dublin which is operated by Aer Arann and which provides point-point travellers and those wishing to connect to long-haul.
As I said earlier I don't see easyJet operating as they go to Belfast and the airports are not too far apart.
In terms of catchment areas, the DfT in their Northern Ireland consultation document estimated within 2 hours of LDY there are 400,000 people, compared to around 1.3 million around BFS or BHD.
Perhaps Aer Arann may step in with a LTN service if FR pull out altogether, since they have links with both LTN and LDY.
LTNman
It may seem odd to buy an aircraft with a derated engine which can't operate at all your existing airfields, but if buying this version means that it costs you less in maintenance and other costs and the aircraft can still carry full loads from most of your airfields (including your main bases and key destinations), but can't from some of your other airfields then why not? Particularly if some of those smaller airfields have runway extensions built which allow full loads, and those that don't - well you just pull out and go somewhere else. As I said, MO'L's not slow in spotting an opportunity to save money and not spend money when someone else will instead!