1. The secondary applicants absolutely MUST also enter NZ by the date given on the visa, to activate that visa, or it will lapse. They could then take the next flight out again, or stay only for a brief visit then leave, if it isn't convenient to settle in NZ yet at that time.
2. Two points to this answer.
a) Here
http://www.immigration.govt.nz/migra...quirements.htm is the summary of requirements for gaining permanent residence. You, the main applicant, must meet all of the top paragraph, but you only need meet ONE of the paragraphs headed
Commitment to New Zealand criteria. The first one, about time in NZ, the one you had quoted, is the one probably most people use, but it's not mandatory (as in, not the only possibility).
b) Once the secondary applicants have activated their visas, IN GENERAL they don't have to meet any conditions - they will get Residence as long as the main applicant does. However, notice that some things can change that, e.g. the breakup of a partnership and/or dependent children becoming adult and their educational and financial status. See here
http://www.immigration.govt.nz/opsmanual/45650.htm RV1.20 b, RV1.20.1, and RV1.20.5.