Quote - ...Officially, does Poser "support" n-gons? If it is supposed to support them, then I think this is a Poser bug in the n-gon scenario...

IIRC, according to DAZ, Poser doesn't support n-gons.  Tris and quads are only, specifically, supported.  It's in their guide to packing up products for sale, IIRC.

Quote - I know Poser is supposed to support tris and quads, so TheOwl's cylinder should work. But ... I'm not certain that it is legal to claim the same shared vertex simultaneously has two different normals on it.

I am by no means a professional.  That being said: When looking at problem areas with tris in Poser rendering, it's almost always either multiple "thin" tris crowded together (some applications exporting objects as all tris really, really screw it up for Poser) or tris that aren't coplanar to each other or that are bordering material zones with geometry not coplanar to the tris. (I think there is an example remark of that in the thread about the Wall and tris/ngons around here.)  Yet, I've also seen models that have been exported as all tris, have material zones all over them and are cloth objects that seem to perform just fine in rendering/preview....   Go figure.

Quote - In the parlance of an OBJ file, is the sharing of a vertex merely a notational convention or is it supposed to semantically reflect an actual sharing of geometry. In case that is not clear, is re-using an existing vertex legal just to save space, or does that have topological implications that must be avoided by duplicating the coordinate in a new vertex?

Not sure if this helps, but... Basically, as I understand it, the idea of "shared vertices" are used to reduce space and increase computation speeds but, in order to be understood, must also have an accompanying index which describes their role in the "geometry" to the video/renderer.