Congratulations on the stag. My condolences on having to fly United for that long of flight. Surprised you did not take the Air New Zealand direct from IAH.
Got the b-class ticket with miles and the IAH-AKL wasn't offered at the times I needed. It would be an excellent alternative and shorter route. I will say United actually did a decent job, with the exception of the check in at IAH. The meals were quite good, airplanes clean and everything worked and ran on time.
With the exception of the IAH check in and a bit of a hassle coming back through SFO, all the firearm dealings went pretty good too. To import a rifle into NZ is fairly easy and I handled it all myself. Basically submit a request online a few months ahead of the trip. Upon arrival in Christchurch, customs was actually pretty picky about looking at the soles of my hunting boots and if there was any blood on my hunting knife. After that, I went to the police office in the airport and they had my rifle waiting. After inspection, verifying the serial number, the officer made some friendly small talk and I paid my $25 NZ fee and was cleared to go. Bringing the rifle back to the US was fairly painless in San Francisco. I have Global Entry and was directed to the CBP office. An officer led me to the baggage area and we waited for the bags to come out. My checked bag came out on one carousel and the rifle case on the oversize carousel. We took the rifle over and I had to unlock the case and CBP inspected the case and serial number on the rifle. All this went quick and easy. When I went to re-check my back with United, a time consuming hassle occurred. United said one of their agents had to take the rifle and me to a TSA check point. This was early Saturday morning and let's just say I ended up seeing much more of SFO than I needed or wanted to. It wasn't even TSA who ultimately checked the rifle. It was a contract version of TSA, I believe the emblem on the uniform was CSA. That took about an hour of my 3 hour layover in SFO. When I finally got done with the rifle, I cleared security and went to an extremely nice United business lounge and was hoping to shower and change. No such luck, as the lounge didn't have showers since it was in the domestic terminal. I was told I could walk back over to the international terminal, but declined. Flight to IAH went well and collected my rifle at the United office in the baggage area and then got on the bus to Park-N-Fly parking and drove home in Houston heat and humidity.