I don't think this would have been possible without butterflying "Cape to Cairo" ambition. I think the only way to make this possible is for Ottomans to be victorious in 1877-78 Russo-Ottoman War, so that Egypt won't be falling into British hands, so no ambition to Cairo. However, to actually what extent shall the impetus for northern exploration be reduced by negating Cairo ambition ? Or would there be no chance for the Portuguese either way whatsoever ?