"First off - you need to do better research. The photo you are referring to of the Viet Cong prisoner being assasinated was taken by Eddie Adams. And secondly, I fail to see how you can compare this to Witkin's photos. The VC execution photo was news, it was something that was recorded while it happened (it was also filmed by a crew working for NBC). Witkin's photos are pure construct's for his own purposes, whatever they may be. These thing's are definitely NOT comparable."
I didn't mean to re-open any can of worms surrounding that picture--and you're right, i know nothing about the story behind it besides what you just wrote. It simply seemed like a convenient example of how sense of authenticity influences our perception of pictures.
It's true that witkin's pictures are staged, but that's not the same thing as "completely fake." staged photography is a tradition dating back to the mid 19th century, and part of its power has always come from the tennuous relationship between what's real and what's constructed. This is one of the reasons its practitioners have turned to photography instead of painting--because of the various implications, however naive, that photography carries.
Still, my answer to the original question, "would it make a difference if witkin used fake bodies," is that I don't know, but that for reasons I've given, I suspect it might. And again, these reasons are are not tied to the ethical questions raised by the work. And again, saying so is not a dennial that these ethical questions exist or that they're important.