Miss Nancy asked "am I correct in assuming that, in the absence of any other light source,
the P7 IBL light doesn't allow materials to exhibit optical properties
such as reflection, refraction, transparency, shadows, et al.?"

No that is incorrect. The only thing you lose with IBL is specular reflections, i.e. simulation (using specular nodes) of the almost direct reflection of a light source on a surface. Why? I can find no reason, other than they were lazy, or they don't know much about computer graphics past 1990. I say this with some safety because I have implement soft and hard specular effects in shaders using images and the sphere_map node. The sphere_map node and the built-in IBL diffuse illumination are almost exactly the same thing. But I digress.

Ordinary object-to-object reflections are not affected by the types of lights you use and in fact lights are not needed at all to get reflections. You may ask then what could possibly be seen then? The answer is anything you include in the scene that lights itself, i.e. uses ambient or alt-diffuse or alt-specular to force a color into the render even without light.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)