John,
I like what I've heard so far, most all conjectures being both faithful to the texts and plausible, and can be quite content in leaving it at that.
I do want to throw another element in the mix, however, and would appreciate help from the Greek-readers here on 2 questions:
1. Is "headlong" here likely to have the primary English meaning of "head
first"? and
2. Can the "fell headlong" construction allow the implication of "with assistance", as in "[his body was cast down and] fell headlong"?
If yes to both, I would find easier to picture the body being removed--in keeping with the law--from the place of hanging and being unceremoniously hurled--as by a couple men holding each side of a sheet--headfirst over a wall or precipice into the field at the bottom of the hill, than to picture the body somehow inverting itself in mid-air.
To me, this scenario also more easily provides the necessary eyewitnesses for Luke's account. While the hanging and final condition of the body would have been able to be seen for some length of time after the fact, the "falling headlong" could not have lasted much more than 1 second, and would be very unlikely to be directly witnessed. However, a talkative temple guard in later years, perhaps, could have given a direct account of what happened.
That said, this is all speculation, and only 3 things are certain:
a) it happened just the way it was recorded,
b) if we needed to know more, we would!

and
c) how undeserved the grace that rescued us from such an end!