Can't help on the best time to grab NZ seats but do know they are very difficult to come by

When you are looking at routes and booking you should look at 2 one-way tickets rather than a return. This gives you some advantages
1) Can have a stopover in each direction
2) Can change the return part (for a fee) even when you have started your journey

the disadvantages are if you need to change or cancel then it is 2 trips and not 1 but think the advantages outweight this

Mileage calculation is just half the miles for the return