So to conclude: we don't know exactly what happened. We just have different ideas that all have pros and cons (okay two are very similar, IMO). Is that correct?

PS: My two cents concerning the style: I was trying to put things on a global Africa map directly at the beginning of the post, when you were describing the first migration streams. And the map came after, with the second migration pattern. Maybe would it be good to put a map first? This would have been helpful for me (but of course this is subjective :p)