PPG still would've come into play, even if we give Kahibah 3 points as Adamstown had a better PPG means they would be going up since all their games had not being played. You say they would've won one more game anyway which is true so that would put them on 75, not only 2 points ahead of Kahibah if they were given the 3 put also further ahead on PPG.
Also Adamstown didn't need to win just one game against Singleton, according to Northern before the weekend their PPG was 1.53 compared to NIAS 1.47 and Kahibah on 1.47 so were ahead of both anyway. It would've made more sense for Adamstown to play no games just to ensure they stay up.
I was told by one Kahibah coach that they knew they weren't going to get in about a month ago and so were just focusing on the second phase now in the bottom tier so much so that they had given teams off from training. Perhaps they could've kept trying to get more wins rather than giving up early on. Northern didn't punish them, they punished themselves by resigning to defeat too soon.