Hi Michael,
The bill seems to be pale (i.e. not red (Shaft-tailed) or black (Long-tailed, even juvenile it seems)), and the legs dark (i.e. not red (Shaft-tailed)). The face is marked (Shaft-tailed female markings are not strong) but not black (like Long-tailed female on the face).
Of the three vidua possibilities in the area, I therefore think this best fits female Pin-tailed Whydah.
We saw Pin-tailed (male) at Mahangu in the Caprivi Strip, so I don't see why you wouldn't see it at Khwai which is not far at all.
But this is just what I would have thought if it were my own photo. Maybe someone with more experience will have a more confident opinion than me. It's difficult to know about regional variation (for example Pin-tailed females in east Africa seem to have darker bills judging from the books, and my experience in east Africa, but the Sasol guide specifically highlights the pale bill of the female in southern Africa.