Hi Colin,
I love this type of discussion!
The rondel angel is intriguing, but first thoughts are that the rondel is a form marked by its flat terminals on the pommel and guard. The Ashanti swords, or what we are looking at from the Tomba/Mende are rounded. Which is not say the idea isn't without merit.
However, as I usually do with takouba, I like to examine things from the angle of practicality and how a form might be arrived at.
Given that these hilts are usually wood (the Ashanti example is somewhat more ceremonial) the shape seems like the most logical form you could come up with when trying to get a bit of hand protection, a bit of balance and a bit narrow enough to wrap your fingers around.
By which I mean it's not a hard shape to arrive at when putting together a weapon. I'm not sure Europeans need to be brought into the equation to explain it. For one thing, we don't have anything predating the Europeans which creates a big "oh" moment when things obviously changed. At least that we know of... The usual unfortunate caveat when dealing with African arms and armour. But just because we don't know doesn't mean I think, that we have to assume some sort of radical shift when Europeans arrived.
I guess my main point is, do we have any reason to suspect it required outside influence to create this hilt form? Personally I don't think so.
More direct European influence can be found for example in this Christies auction from a number of years ago.
http://www.christies.com/LotFinder/l...jectID=4515427
Just my two cents/pence. :-)
Iain