Well, I got to try and use a true HDR image to light my scene, and it was easier than I had though.

I wondered for some time as to how, I could lower the HDRI's brightness, and although I'm not sure this is the way to do it, I simply lowered the "color gain" in the attribute editor for the image node (the materials incandescence attribute)

I tried making the hdri file darker with the freeware HDRShop, but the tools there seemed only to produce low dynamic images like jpeg, tiff and the like.

So the "color gain" was the only attribute that gave me any results. Please correct me if I'm wrong here, or If there is any other way to darken you hdr image, to lower the light in your scene.

I am satisfied with the final rendering, but although I previously stated that this rendering would be done on the basis of my earlier tests, I did not follow up, and unneccesarily set the minmax value too low, causing the rendering time to drag out to over 18 hours. I guess the rendering time could have been shortened by skipping the directional light in my scene (with special shadow attributes (1.5 deg light angle and 15 shadow rays for smooth shadow-borders)

Next time I have to figure out how to combine FG and GI.