You should have your pick of any available UA J seat within the next several days, but unfortunately reaccommodation on any other airline in J, including alliance partner NZ, is quite unlikely. You can keep HUCAing to see if you'll find an agent who'll put you on the NZ flight in J, but I wouldn't count on it.

Yes, there are arguably bad and unethical business practices at play, but until/unless there's a law requiring UA to rebook you on any airline in J, it will likely get away with its current practices.