Driving Akl to Welly with a 8-9 month old

We drove Tauranga-Welly when Hannah was 12 months, she was as good as gold.
We stopped when she woke so she could stretch her legs and have a snack/drink/play. Worked really well.
She slept more than she usually did during the day, so I was worried it would mess with her night sleep, but she was fine. She went to bed a little later, but that was probably more from excitement about seeing family and being somewhere new, and slept just fine.

Regarding flying, I've flown with Hannah domestically about 6 times. Each time the car seat and pram were "free" and Hannah also got her own allowance, even though she was sharing my seat. So I packed all our stuff in one bag (my allowance) so, just in case there was an issue, Hannah's allowance was used up with the pram or carseat - leaving only one extra piece of baggage to try to get on for free or pay for, depending on how full the plane was. I've never had to pay excess baggage.