Honestly I don't think the territory will go to any single power.
That said, Belgium never coming to be creates butterflies. We might not see a fully realised German Empire as one example, so it's hard to say who would end up in Africa.
If we assume the unlikely scenario that the rest of history goes similarly and the same powers are involved in Africa, then my assumption is that the Portuguese gain the Kikongo speaking areas at the mouth of the river, the British grab Katanga and the east to bypass German Tanganyika, and the French get the rest.
What made Belgium unique is that its neutrality was internationally guaranteed by treaty. You can't say the same for countries like Sweden or Denmark, or the Netherlands, or even the United States. Because of this neutrality, Belgium was seen as a reasonable compromise between the competing great powers (Britain, France, and Germany, who all had interests in the region; France in Gabon, Germany in Kamerun/Tanganyika, and for the British Cecil Rhodes went as far as Katanga IIRC as part of the British South Africa Company). Belgium was also industrialised, and in need of raw materials, which was the ostensible reason for colonies in the first place.
Not to mention Leopold had grand ambitions of empire. You can't say the same for Sweden, or even the United States (which was still pretending Puerto Rico, the Philippines, etc. weren't colonies).
