2. Currently (July 3) there are no seats available in PE HNL-AKL but I did a "phantom" booking to see if there was space available in PE when I got the "seat assignment at the airport" message in June. There were several seats available then.
I suspect you have answered your question there. Looking at NZ9 for July it appears heavily (over)sold nearly every day it operates, so I suspect that NZ is withholding advance seat assignments to give flight editing flexibility to move people around to keep groups/families together, and clear upgrades as needed.