The 12 month period begins when you enter the country so you will be able to drive on your licence for 12 months from the latest date you enter NZ.
Yes, and the 12 months re-starts any time you leave and come back in.
It sounds like you have covered most of it nms.sun3. With regards to IRD numbers, we also applied for ours while still in our home country and did not receive any e-mail communication. However a few weeks later we received letters from the IRD at our home address in our home country. Not sure what address you gave up, but maybe check if it was not delivered to that address.
The information and requirements are all on the IRD website. Have a look here:
https://www.ird.govt.nz/how-to/irdnumbers/?id=footer
Yes... but the NZ police come down hard on the many people who keep putting off and putting off converting properly to a NZ licence, if once they are caught driving AFTER 12 months after their last entry. Don't ever be tempted to think 'she'll be right' and 'I only need to go just round the corner' and 'it's only been a year and three days and I'm off on my trip to (wherever) tomorrow'.
I did my IRD at Kiwibank/NZ Post the day I arrived. After a week I rang in to get the number so I could get paid at my job.