2 types: High frequency ATRs to the mainland, plus C208B's inter island and ACI-SOU is the solution. The latter can be done with careful routing, altitude and synthetic vision options. 1 P&W PT6 is far more favourable than 2/3 piston engines: locals have idolised the BN2/3 with single crew for 30 years, so single point of failure has been the choice for a generation. And I'd choose 1 PT6 over a fully laden piston variant every time. Better still, 1 PT6 and 2 crew.
IFSD rate is quoted as 1 per 12.4m flight hours.