Here was the first render, with what I thought were very aggressive render settings. I used the D3D Render Firefly dialog where I can control things directly. I had IDL bounces=6, samples at 6000, Irradiance Cache set to 90.

The env sphere HDR value was set to 16. (I'm not in a fish bowl - with the relatively small angular cross section of these windows, I needed four stops.)

After two hours I got the usual ridiculous light leaks and splotchies. But, at least the lighting looks about right.

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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)