I think whether they sell out or not would depend entirely on the price point of the tickets, which is why they need the time to gauge interest from governments, broadcasters, sponsors, and corporates to see just how much they can make from them in order to subsidise the price of the tickets.
PNG would be fine to host at least a few games, but the rest are a bit of an open question as to how profitable they'd be, if at all, and whether they'd be commercially competitive with simply hosting those games in NZ or Aus for that matter.
Sure it's a bit different to internationals, but I have it on good authority that for the most part both Moana Pasifika and the Dura's games that have been played in Samoa and Fiji respectively have run at significant losses despite drawing strong crowds (same is true of the Hunters BTW) and the IRL simply can't afford that when the WC is their only significant source of income. So like Super Rugby has done for MP and the Dura, they'd need to find ways to eat those costs if they're going to play games in those nations, and that's what they need to gauge interest in.
In the case of Moana Pasifika and the Dura multiple government agencies from NZ, AUS, Fiji and Samoa themselves, World Rugby, and other sponsors and the like, all chip in to make Moana Pasifika and the Dura viable, similar support would have to be sourced for any RLWC games held in the PI's and it'd be a big ask to get that all done in multiple nations in under two years whilst still organising the rest of the competition.
Ideally they'd also get repair work and upgrades done on Teufaiva Sport Stadium before they even started seriously discussing Tonga, and that definitely isn't going to happen within a couple years.