TBF progress really isnt being made very much in most of those countries (PNG being the exception), just that those countries are benefiting from migration to NSW/Qland/NZ where the kids of the immigrants are playing the game in a very developed system. How many of Tonga or Fijis team were born in their country and played Jnr RL there at any age?
Fiji have been producing players for the past few years, from either code for the NRL. If you follow the Fiji NSW Cup Facebook page they often display photos of current Fijian Sevens players and what Fijian Rugby League club they had previously played for.
In an interview after the All Blacks massacred the Tongan Rugby side earlier this year, the Tongan captain said in and after match interview that Rugby League has become the most popular sport in Tonga.
It's clear as day that unless you have a team feeding into a premier competition like the QLD Cup or NSW Cup, you will struggle to produce professional players. Look at PNG, since the inception of PNG hunters in 2013, after the 2013 RLWC the Kumuls they have won 8 of their last 12 tests.