Have you tried speaking to a (decent) travel agent? They might be able to ticket everything together, but I suspect you may end up in a similar situation with low SP earn for AirNZ. I think my example above was ticketed via Swiss, which wasn't an issue for me as I was crediting to another programme. I only use AirNZ for domestic / trans-tasman mileage.
Re separate tickets I assume you know all the details re your responsibility if connections are missed etc. Flying that AKL-PVG-LHR route has had big delays before due to pollution? in China (
generic article) which causes pilots to go over hours and needing new crew etc. I wouldn't personally be booking separate tickets unless I had a decent (overnight+) chunk of time between the flights. When I stop in Asia for a day of meetings I am usually on the same ticket so I don't have to bother with picking up bags and checking in again and getting boarding passes etc, which is likely a hassle if you are wanting to make the most of your limited time on the ground.
I assume the SP earn is only on AKL-PVG leg, so I wouldn't imagine the difference was that much?