Brecht Van Lommel, the original author of Cycles, told me that I should set the sun size to zero (in the Blender scene), so that Cycles will get through the windows correctly. That was the question I had in a previous post, and indeed it fixed the problem:
Here, for comparison the rendering I got when the radius was not set to zero:
And the resulting imf_diff output (in this case without the false colors):
The updated Blender file can be downloaded from the repository.