The MasterCard discount applies for award travel on SAS. No code-shares, has to be SAS themselves. Only for points bookings, not regular tickets (so not for points+money).
And you're correct, a business award to Auckland will be 180,000 points with an Amex 2-4-1 voucher in business, or 240,000 in first. Which would be the best possible way to use the 2-4-1, and the best possible discount for that trip, both at once
(and the MasterCard discount wouldn't be usable here, since SAS don't fly to Auckland themselves)