I don't think Milne should take the new ball. He's to be used as a shock weapon, after Boult / Southee have used the new ball.
The idea of pure pace is its still a threat even with an older ball.
You're also saying essentially that if we play Milne we are tempting the fates, and that if he makes it through the test then it's likely Boult will break down. I'm not sure it works like that. I'm not sure that tempting the fates isn't just old fashioned superstition.
I can see the arguments for not playing Milne, still I would like to see us play him. I always like to see us go against the ingrained conservatism of NZ cricket, the baleful thing that Rixon fought so hard against when he came here, that "oh we mustn't do it because it might backfire" tentative, wussy stuff. It was him he pushed to get Vettori into the team when everyone was "oh, you mustn't put a young unknown spinner in the side, it isn't the done thing". It's the fear that NZ will look silly, also that everyone here at home will boot the boot in to whoever makes the (gutsy) selection, using the knocking machines gift of 20/20 hindsight.
What have you got to lose is taking risks to pursue victory? Absolutely nothing. If Milne doesn't come off, we have the resources to cover him. But the upside could be huge. If he did come off, it could make all the difference.