Welcome to flyertalk!
Just some answers:
- You need to find flights on Star Alliance carriers - not many options in the south pacific. Check the staralliance website for possible routings.
- You are limited by the number of miles total, number of coupons (16) and some others (like crossing the pacific and the Atlantic once and ending up in the same country/area (the rules are more specific, this is just as a starting point).
Date changes are possible at no cost as long as there is space in the respective booking class. Sometimes this can be difficult to find, even if the flight has lots of seats for sale.
- The price the staralliance website gives you is accurate, the fares are good when you make use of many stops and use up the coupons and miles. This can be significantly cheaper than multiple one ways, especially in premium classes. If you have a relatively simple itinary with not many stops one way tickets are often cheaper.
Do a search in this forum and you will find much more detailed information.