It's also possible to have codeshares on airlines that don't earn any miles on UA at all. Sometimes a codeshare is sold just to make it look like UA can get you all the way to somewhere they don't actually fly. Not sure if they have any currently, but it's possible.

Codeshares have absolutely nothing to do with earning miles. If Air New Zealand operates the flight and you've given them your UA MP#, you're going to earn the miles that UA gets for NZ flights. Regardless of whether that NZ flight has a NZ code, a UA code, a Thai code, a Lufthansa code or whatever. It'll be based on what the operating airline (the "metal" around here) has in their own reservation system for it for that booking class and what that NZ booking class earns in United Mileage Plus.

Which by the way is different from how AA AAdvantage works over in oneworld, where codeshares do matter. In Star Alliance programs it's always the metal and the operating carrier's "real" booking class that matters.

Not always true that the codeshare costs less. In some cases, US codeshares on UA metal are less than the same UA flights. Mix of LH codes on UA metal one direction and UA own code on return often price out cheaper than all-UA on the same flights on TATL, due to oddities with how LH and UA do the surcharges differently.