Re: Two questions
Oh, that "preview"
I was clicking on the Preview tab rather than the Preview... button .
mea culpa
Ya get that 
My bet is that this will be 'fixed' in the next update.
No bet - BTW, if you go back into |Page|Setup| it reverts it to "0"...
If you try this technique for Page Numbering it will cause the page numbers not to display.
Flaky. The Preview... button shouldn't bypass routines like that.
True, though it does suggest that a simple programatic change would allow negative bar numbers. Thus it becomes a policy decision...
In reality, in NWC 2 there is a very easy workaround for bar numbering - start from "0" and then set however many barlines you need to be effectively less than "0" to be un-numbered...
As for page numbers, I personally don't see any advantage for negative ones, though that doesn't mean there isn't one.