Obviously you’ve thought this through, and it’s your choice.
However, (unless I’m being thick) one of your objections to SQ is that there is a change of plane in Singapore. But if I’ve understood you correctly, you have a change of plane in Auckland with Air NZ. So what’s the difference? You still have the same risk of lost bags and missed connections……
You say that you can check in at Christchurch for the international return leg at Auckland; if by that you mean you get boarding cards for both legs of the return flight you’ll be able to do the same with SQ….
Fair point about the layover at Singapore on the way back, but Changi is probably the best airport in the world in terms of providing ways for you to pass the time between flights. Movies, swimming pools, foot spas, gardens and more are available free of charge. In fact for a 7 hour layover you’ll be able to take a complimentary city tour……