Q1) Is it only necessary for principal applicant to be in NZ as a resident for a total of 184 days or more in each of the two 12-month portions of the 24 months? Don’t non principal applicants need to be there too?
--> Yes, only the principal applicant needs to be resident for the 184 days in each of the two years. See the operational manual RV1.20.
The only requirement for the grant of a permanent resident visa to a non-principal applicant is that the principal applicant must hold a permanent resident visa, unless: the non-principal applicant is excluded by the provisions of RV2.1; or the provisions requiring a secondary applicant to be assessed independently of the principal applicant apply (RV1.20.1 to RV1.20.20); or
the non-principal applicant has never been in New Zealand as a resident (as required by RV1.5(d)).