its difficult, significant development requires the Livingstone Falls to be bypassed by railroad (prior to this the only way to get goods in or out is via human porters, expensive, often fatal, and dehumanizing). No native state could afford to do this, and any external power is going to want a return on this investment within a few years and probably pursue unpleasant measures to do so! In other parts of Africa such infrastructure investments weren't a problem , as you could build a little bit, make some money, and then build the next bit - here you need a huge cash injection to get the 200 kilometres of the Matadi-Kinshasa Railway (and remember you need human porters to shift all the materials!) done in one go.
I think the best bet is for Portugal to get the Congo, as they're too poor and overstretched to properly oppress the place, delaying 'development' into the 20th century when some diseases are solved and rampant brutality in colonies goes out of fashion somewhat.