So I have looking through old newspapers to see what the various claims were.
When Portugal opted to seize and annex the mouth of the Congo in February of 1885 … at the time it seemed like the negotiations were going to fall apart … they "demanded the whole of the territory on the right bank between the French frontier and Tehiloango down to the Congo and eastward to the fourteenth degree of longitude."
The claims of the other powers involved however were extremely vague, in the sense that they had an interest in the whole of the region but did not necessarily want to colonize it or had the means to do so at the time, except for France that is. The French at this point did not recognize the International African Association and were actively making treaties with local entities, pushing their colonial frontier further and further into the interior. If allowed they certainly would have continued their expansion, but the British may well have endorsed Portugal's claims to the mouth of the Congo River to prevent a French monopoly on trade in the area, assuming a Free Trade area is not later negotiated amongst the Great Powers involved. You'd inevitably then have Germany coming into the Congo by way of German East Africa, and then the British by way of Katanga and Rhodesia. A conference would need to be called sooner or later, least it become yet another potential powder keg on par with Fashoda.
The greatest irony is that it is rather likely that even without the International African Association or Leopold an "independent" Congo would probably have been formed; there had been discussions at the Berlin Conference revolving around the creation of a "European Monarchy in Africa", I believe in no small part influenced by the experience of the White Rajahs of Sarawak. This Kingdom however would only have loose control over its own affairs, the nation being neutralized and forbidden from having a formal military, it's land sold to any number of Trusts willing to pay the associated costs.
It'd be better than the Congo Free State, but by how much I can't say.