Ok Deltasix, it did sound very simliar to their plans a few years back.
The 228-212 is perfect for the LHI/NFI ops and operating out of Willy puts you that much closer to LHI (though a think Port or Coffs are closer). At my guess you would get 15- 17 pax and bags with enough few to get back to the mainland.
I guess the only other smaller type would be the B200 or if you can find one a B1300 (Beech bulit less than 20 for Messa). Otherwise it's the Dash 8 , D328-110/120 or the Dash-7 (if you want big).
When you calculate the required fuel to return to the mainland, remember it has to be calculated based on the worse case, which in the D228 case could be if you got to LHI, couldn't get in due wx etc and then you couldn't retract the gear and had to fly back with the gear extended, therefore reduced TAS and possibly higher fuel burn per hour.
All the best with your plans, Qantaslink has had LHI to itself for far to long.