Doing a search here http://www.immigration.govt.nz/templates/custom/SearchskillshortagesPopup.aspx?NRMODE=Published&NR NODEGUID={3B3850BF-FB16-43DF-9CCD-FE0342BFB73C}&NRORIGINALURL=%2fmigrant%2fstream%2f work%2fskilledmigrant%2fLinkAdministration%2fToolb oxLinks%2fessentialskills.htm%3flevel%3d1&NRCACHEH INT=Guest&level=1 for Mechanical Engineer, I've found the Engineering section, where the requirements in the last two columns (for LTSSL, and for bonus point for qualifications in an area of absolute skill shortage) are as follows.
Here
http://www.ieagreements.org/Washington-Accord/FAQ.cfm is how to proceed to find out if your degree(s) are or are not Washington Accord recognized. As you'll see from the requirements above, if they're not, the only way to be in a position to claim the points you did is to get the IPENZ letter, since both your degrees have turned out to have a generic outcome. That simply means that their content doesn't have close equivalence to any existing NZ degree. And that is why INZ, who aren't expected to know about the ins and outs of all professions, want evidence from the engineering experts to say that your qualifications would indeed be valuable and useful in the NZ context.
You can't get past this point without EITHER showing Washington Accord recognition, OR supplying support from IPENZ. If you want to get to live and work in NZ, and not waste the effort you've put in already, you have to take this next step - up to you to decide if it's worth it.
I agree that it seems rather odd that the CO didn't raise this till this late stage - maybe she only then noticed that your IQAs were giving generic results. However, annoying though it must be, when a CO says, 'Send x,' you have to send it.
Good luck.