1

I have spent many a night in Auckland Airport when I have an early flight out. I live close by but it saves my husband getting up in the middle of the night. Where you stay is all before going through Security. You cant do that till you check in and Auckland doesnt have early check ins for any airline.
Just a matter of following the arrows if you want to walk between the 2 terminals.

2

I have done it last year.

I had a early intl flight out from AKL but the domestic flight into AKL was the previous evening.

You can sleep area in the Intl Airport such as the chairs in front of the checkin counters, outside the arrivals section or upstairs where the cafes and McDonald is located. McDonalds is not 24hr hours.

Only once you have checked in, can you get thru security and sleep inside there.
That happens maybe 2 or 3hr prior to take off.

The observation deck is before security. It is one floor above checkin counters where the cafes and McD is. Then you go to the back and take one floor up again with the escalator. I think some the airport lounges are around there too .....

3

Yup, I took the advice from that website and it worked out fine.

We were on the first flight to Wellington the next morning, but since Domestic closes overnight, we stayed late in the city for a few beers, then got the bus to international, slept there for a few hours, then got the transfer bus across to domestic for our flight.

Was surprisingly comfortable as well.