Only one of the OP's listed missions has any overwater segments, and that BUF-NAS mission has plenty of contingency options in the unlikely event of an OEI.
Not knocking the DA-50/900.... In this case, it's not necessary to feed a third engine for the listed mission requirements. The OP did state that capital and operating costs were a strong consideration in the selection process.
I am looking for the cheapest plane to purchase and maintain.