Lovely post, Mr Edgeler.
I am not sure I agree w/you that National would be "quite happy" with MMP being retained.
If you think (as I do) that the four current Parliamentary parties likely to survive long-term are Labour, National, the Greens and the Maori Party, then it follows that the Left has much more to lose from a dilution of proportionality and a move to a majoritarian voting system.
The recent Australian election showed this quite well: the three parties to get over 3% of the primary vote were Labor, the Coalition and the Greens. Held under AV, they had a hung Parliament. The same voting patterns under FPP would have given the Coalition something like a 15-seat majority, and under MMP would have given Labor/Greens something like a 10-seat majority. In a situation where National does not have serious competition for voters on the Right (and Labour does on the Left), majoritarian voting systems serve its interests very well indeed.
What you say about the Maori seats under FPP is very interesting. Effectively, only in landslide results (such as 1975, 1984 and 1990) would the Maori Party be robbed of the balance of power...