Even 25 years ago Manus didn't need a 110 seat aircraft, much as it doesn't need a 150 seater now. Back in the TEA days I suspect they were asked to look at Goroka, Hagen, Madang and Rabaul. Madang was short back then for a -200, prohibitve for a -700. Rabaul is gone and Tokua is dirt (they don't make a gravel kit for NGs) And I can just see the Swiss eyes roll when asking the question " What is the go around at Goroka". Hagen- can't see any real objection back then other than it didn't have any IFR approach. Despite the Swiss living amongst mountains they didn't like the PNG mountains. I remember a series of technical problems (real or imagined) plauged the aircraft and it only lasted a few short weeks.
Nadzab with contract crews? Big issues there and PX best avoid that.
Having said that I think it is long overdue that they operate a 150-180 seater. Maybe after deciding not to buy MBA they saw the marketshare they have lost on the BNE-POM route and had to compete with like equipment.