Hi Andrea.

FJ fly from YVR to NAN 3 x weekly - if you're in SEA, that's a good option to look at as I believe their space available on that route is always not too bad at all. The flight routes via HNL so if you can get to/from HNL easily enough with your carrier/partners then the HNL-NAN section of the YVR-NAN flight should be okay.

NZ fly nonstop from LAX to NAN as well - that may be another option for you if you have ZED agreements with NZ.

Mid September is Spring school vacation break in Oz so the flights to NAN may well be heavily booked - another consideration.

As far as the infant ticket goes, depends on the interline agreement between your carrier and the carriers you are choosing.

Not much help, but more info to chew over!
Cheers.