Quote - Is this stuff documented anywhere?

The only comprehensive documentation of Poser library files that I was aware of was the "Unofficial CR2 File Specification" at "www.kuroyumes-developmentzone.com", unfortunately that resource no longer seems to exist on the site.

Quote - Are the contents of these .pz2 files determined by DAZ, Poser or both?

The language of 'Poser Script' used in Poser library files is determined by who ever owns Poser this week. The actual contents of any specific pose file are determined by the application that wrote the pose file.

Quote - I suspect some of the anomalies that I've pointed out are simply oversights or mistakes by the vendor.

I share your suspicion.