Thank you very much for the interesting pictures!
It's very interesting that the bows Hadza were seldom unbraced. I belive there is actually a good point in it: I think that these bows don't have any nocks that hold the string. The ancient Egyptian bows didn't. The strings were attached to the bow using knots and windings. I also did it like that and I can say that bracing a bow like that takes some time and doesn't work well if you are in a hurry. But there seems to be a difference between Hadza bows and ancient Egyptian bows: The Hadza bows show a lot of set from being braced for a long time while the preserved Egyptian bows don't. This rises the question, if those ancient Egyptian burial bows were actually used before or especially made for that purpose. Or maybe they just unbraced the bow each time so they didn't take much set.
I just had another idea, why those bows were/are made with deflex tip regions. I think that the tied knots don't slip that easy, because the direction of load is closer to 90 degrees.
With regard to hornbows, I don't know any Egyptian examples with the same shape. According to my knowledge they were brought to Egypt by the Hyksos, a nomadic tribe, before the beginning of the new kingdom. These bows were angular bows with deflexed grips and reflexed bow limbs. But the shape of those antilope horns might fit perfectly for this kind of bow.
lonbow
here's a picture of Egyptian bows. The upper one is a selfbow and the lower ones are composite bows. Their shape is different.
http://www.griffith.ox.ac.uk/gri/9bow.html