Whoa, okay - I figured out the issue with the render artifacts! I swear I tried this very early on in trouble-shooting, but n-gons (well, some of them) were the problem. Simply tripling them worked great.

... but only if I do with triangulation withing Lightwave. If I export from MoI with tris + polys, LW doesn't like it.

I think perhaps the problem is my platform of choice is extremely picky.