wouldn't it be better if Eagle, Nelson and Cook were all flying the same equipment, to the same SOPs
I have no doubt it will go that way (in fact it has already happened withing the regionals). Just as I said previousely, things like this can't happen overnight (that's why Air NZ didn't just do away with airpoints all together - although I'm sure they will).
It looks quite likely that Air Nelson will get ATRs, bringing the regional fleet to 27 (I think) ATR, 16 B1900D. When the Mount Cook Airline Training Centre (TM) is established (when the sim arrives), I can see SOPs being standardised between the two sections.
If you're keen on instructing, there's a large group of Easyjet Cadets that I'm sure you could contribute to (not to mention the many aero clubs and flying schools that are finding it difficult to hold experienced instructors).