I’m not sure how anyone could have thought this unless the mouth of the Congo was undiscovered at the time, which seems unlikely. Isn’t it usually sources, not mouths, that elude discovery? I mean, it’s much easier to drift down a river than to paddle up it. All Stanley had to do to (dis)prove that the Congo and Nile were part of the same system would be to get into a boat anywhere in the Congo and drift downstream. If you reach Cairo, then the Congo’s in the same system; if you don’t, then it’s not.
I’m not following. Anyway, the Congo was know to lead backwards to Lake Albert. If the Nile also originated in Lake Albert then voila. Fortunately, it doesn’t.
The Congo goes over a series of serious waterfalls just short of its mouth, preventing oceangoing ships from getting very far inland. The other European colonizing powers didn’t want to deal with all the land transportation around them that was required, leaving the area for Belgium, which dealt with it by enslaving enough locals to get past them.
I think you have the wrong lake. The Nile does flow via Lake Albert, and before that it is fed by Lake Edward (aka Lake Rutanzige), to the south. The Congo is fed from Lake Kivu, about 70 miles south of Lake Edward, with the watershed between the two river systems defined by the Virunga Mountains, between these latter two lakes.
See the drainage basin map at Congo River - Wikipedia
Map of the divides in NA:
#/media/File:NorthAmerica-WaterDivides.png)
Brian