I don't know all the answers to your questions but my experience has shown the following:
1. Upgrades offered at check-in are normally cheaper than if you do them via the call centre in advance. From my experience if we call to do an upgrade from Honolulu to Sydney they will want $800. If there is availability at the airport they will sell the same upgrade for $500.
2. You will earn no additional points for the upgrade, points earned will be for economy class travel. Only benefit would be if you have a Hawaiian Airlines credit card you would earn points that way.
BTW will you be able to checkin online the day before as won't it be an international flight? Only reason I ask is I don't think I have ever been able to check in online when I go to Sydney.