Zatanna! Magic! Lucky Flash misfire!

I posted this image of Zatanna from my Megacon 2014 Orlando shoot. It was well received but begged some questions I am happy to answer here.
It was a happy accident. When I arrived at the convention center, other photographers were complaining about the harsh light in the south-facing concours. I had set up initially with one flash in a saberstrip to compensate and provide the main light and provide a dark background rather than the grey it naturally is in ambient light. I shot a few shots with this model but the flash on one occasion didn’t fire as I had fired a couple of shots too quickly. Later when I reviewed the images in Lightroom, this image was grossly underexposed and I nearly deleted it. Instead I pressed autotone in Lightroom, and because I had shot in raw at a low ISO, there was enough ambient light to resuscitate the image. It was apparent that the large window light the model was facing was more than adequate to produce a flat lighting that suited her look.
Post processing to Original straight out of camera added 2.5 stops of light in LR. I just did basic simple edits with adjustment brush to tidy her face and hair but no other major adjustments, added sharpening (49),a little denoise (44) then Vignette -62, to finish.
Specs:1/160, f7.1 iso400, D600, no flash fired (actually SB600 failed briefly)
Pano of lighting after the convention completed showing position of photographed Grey background behind the blue figure. characters were placed about15 feet away from back wall. I cloned out the two thin lines on the background in each photo.