I like the simpler scene approach using different objects. That could differentiate between an object-specific problem or something more systemic in the rendering chain - I did have an earlier problem with recursion.
I'm happy to note the error in rendering reflective objects turned out to be fairly simple. The calculation of the reflectv component of a comps data structure in prepare_computations was incorrect. Simple enough, and obvious after the fact! There's a bit of a speed penalty as a result of the extra reflection calculations, particularly with the spheres scene for Chapter 11.