I agree with all of that except that I think that in southwestern Yemen there might have been some East African admixture or let's say gene flow back and forth between Yemen and Ethiopia/Eritrea from very ancient times as well as in the modern era. Without ancient dna I don't know how it could be quantified.
See: Kingdom of Aksum
https://en.wikipedia.org/wiki/Kingdom_of_Aksum
Still it seems that the majority of the admixture may indeed have taken place post the slave trade.
There are also "ethnic" distinctions within Yemen. There is a separate "untouchable" caste of uncertain origin, and there are indeed "mulatto" looking people with relatively recent admixture who are subject to their own kind of discrimination. I doubt that the Yemenis who were involved in the Islamic conquests would have looked like either of these groups.
https://en.wikipedia.org/wiki/Al-Akhdam
http://www.aljazeera.com/indepth/op...ladeen-struggle-equal--20144415253333236.html
These are some plates of "Yemeni highlanders" I was quickly able to access. I'm sure that Moesan would know of more. I would think perhaps they would have looked more like this? The skin color is deceiving; everyone looks darker on these plates, and this is exposed skin. They don't look at all SSA.
They are also different from the "Arabs" further north or even Arabs like the members of the Saud family. Both the Yemeni type and the more perhaps "Caucasus" type would have been involved in the conquests.