Wow gwfr glad you didn't get hurt badly!
Any chance you've got pics of the cracked liner? - would be good to see if you do.
It sounds like the helmet did it's job, since they are designed as one-shot devices. It's very interesting that it sheared where the internal visor is - mine isn't at hand so I can't look at it, but is there a recess for it? If so and it sheared along that line it would make sense... but we need pics man!
Can't speak to the reasons Nolan doesn't seek DOT certification (probably financial) but when I researched the DOT vs ECE rating before purchase, I found that ECE rating is actually considered a more up-to-date and somewhat superior rating than DOT. Google ECE vs DOT rating and there are several good articles on the subject. Nolan helmets carry the ECE R22-05 rating which is what made me feel comfortable going with one, as well as their established reputation. A lot of folks are buying the inexpensive off-brand DS helmets here, and though they may be fine, I just preferred an established and well known manufacturer.
Would be great if you have any pics of the interior and exterior to share with everyone
And sorry to hear you have twats in NZ too but glad you're ok man