I don't see any evidence here that the female lines move around. If the female lines were rearranged wouldn't the mtDNAs be spread all over Melanesia instead of isolated to islands and even sections of populations? The total genome of Melanesia may have a lot of diversity but apparently at any particular local it's not diverse.
OK--I see your point. But in Africa, they take the high level of diversity as meaning that they are the oldest populations, but if as you say the Melanesians are the farthest from the Africans, it does not make sense that the Melanesians would be a very old line.