ithinkso,
Why wouldn't Our Airline use Nauru as as alternate? It is part of their network and, looking at their schedule, if they missed out on Honiara and went to Nauru then at least the show is somewhat back on the rails. The schedule shows them returning to Honiara about 10 hours later on the way back to Brisbane so any Honiara passengers wouldn't be drastically inconvenienced. Sounds like a perfectly good call to me considering their particular operation.
I assume the Our Airline aircraft has standard 16 tonne tanks so, if Capt. Moonlight's figures are correct, then they should burn about 8 tonnes on the trip. That leaves around 8 tonnes for approaches and a diversion. Around 90 minutes to Vila would burn a bit bit less than 4 tonnes so wouldn't they arrive there with a bit over 3 tonnes? That is more than the 30 minutes you suggest so you figures look a bit dubious. If the operator you went to Honiara with arrived with 4.5 tonnes then I would be concerned they were cutting it too fine. You know, " an airspace above you, runway behind you" type of thing.