What evidence do you have that GLA-SNN or PIK-SNN doesnt work by itself? It wont work with Edinbrugh, as was tried by EI, due to its extreamly high fares when compared to Ryanair. Two 72 seater flights a day gives 144 seats at roughly 3 to 4 times the fare at what Ryanair could do.
Ill give SNN-MAN as an example.
The cheapest return fare I ever found with EI regional was €80, and that was very rare. With Ryanair, it often can be €20 return if you travel TUES/WED/THURS. The manchester loads reflected this, with a doubling in passenger numbers from DEC 2013 to DEC 2014.
Im sure that a 5x/6x weekly service would work year round, I'm even starting to convince myself that a daily service would work.