Nice going Mgoddard, your ID process looks good.
Yes, it is indeed a Yellow-billed Kite. The all yellow bill is the dead giveaway. Some birds, but notably raptors have, what is called a cere or operculum, around the nostrils. This is the fleshy area that you see at the base of the bill. With most raptors this cere is yellow or sometimes red and in few cases grey. Yellow-billed Kite is the only bird in our region of which not only the cere but also the bill is yellow so, while the rest of your observations were accurate, they were not even neccessary.
The amount by which the tail is forked can vary but your bird still shows somewhat of a fork, something you wouldn't have seen on Tawny Eagle, Steppe Eagle Walhberg’s Eagle or Steppe Buzzard.
Honey-Buzzard is considered a national rarity so it is not really a bird that should come to mind first when you're working on an ID. It wouldn't be strange for you to see a couple of hundred Steppe Buzzards before you see your first Honey-Buzzard. Apart from the statistical unlikelihood (is there such a word?), there are other features that also rule out Honey-Buzzard. The yellow cere for one is a deal breaker. Honey-Buzzard is one of the few raptors with a grey cere. Honey-Buzzard also has a very distinct undertail barring pattern and a distinctive head shape (which reminds of a Pigeon).
Your observation of the extent of the gape is accurate and that does rule out Steppe Eagle and Lesser Spotted Eagle, leaving Wahlberg's Eagle and Tawny Eagle. I can see why you have doubts about the legs being covered with feathers or not. I'm not sure what to make of it because the bird's right leg does seem surprisingly exposed. The bird definitely is an Eagle though and I suspect it has something to do with wind blowing the feathers open. The all brown undertail is also a strong indicator that the bird is not a Buzzard.
OK, so we're left with Tawny Eagle and Wahlberg's Eagle. I think this is a common ID challenge that birders end up with as both of these birds are very common (especially in Kruger). The differences between the two species are also more in their structure really than in their plumage and this is something that takes a while to appreciate. I have to be honest that my first thought when I saw the picture was Tawny Eagle (and I alluded to that in my previous post). The relatively long tail then had me revise my opinion to Wahlberg's. But finally I think I'll settle for Tawny because of the relatively heavy bill (compared to Wahlberg's that have a little more dainty bill), the contrast between the wing coverts and darker primaries and the fairly baggy appearance of the feathers surrounding the legs.
Try not to get too caught up in trying to establish the colour of these birds. Large raptors can be so variable in plumage that it is often of little help to know just how brown a bird is. The picture doesn't show much, although you did observe the gape correctly, but based on the general impression that the bird gives, I'd go for Tawny Eagle again.
The identification of Aquila eagles really isn't a trivial task and you're allowed to struggle with them
. I think many birders don't appreciate just how difficult they can be and try to ID them too hastily.