I don't believe that the Congo would become an international pariah until perhaps a later period. The majority of condemnation came from Britain and the British press and to a lesser extent the American Press. The Congo hosted various Christian missionaries from an array of countries, many of whom complained to their respective governments before 1906 (the year Leopold agreed to surrender the Congo). The response of the government of Sweden-Norway that the state of the Congo was unremarkable as such conditions were typical of colonies as reported in Les relations entre l'État indépendant du Congo et l'Italie by Liane Ranieri (1959) was typical of the prewar international order.
Apparently the governments of Denmark, the Netherlands, Russia, Spain and Portugal paid scant attention to the atrocities in the Congo when missionaries complained. The problems were brought to the attention of the Kaiser, but did not bring up the issues upon meeting with Leopold, with these issues being considered a low priority. The most likely outcome would have been a partition with funds being given to compensate Leopold.
There seems to have been talk of a partition, but then great power rivalry falls into play with Germany, Britain and France failing to agree over the partition. France would not want neither France nor Germany to control the mouth of the Congo, though all the powers might to Portuguese control over the region around the mouth of the river to Portugal (perhaps all territory south of the Kasai River) provided that Portugal agree to free navigation and commerce. Portugal had at one time claimed the Lulua River as the Eastern frontier of Portuguese West Africa, and they might get that too. These slivers of territory would have buoyed the popularity of the monarchy at least temporarily as it would be seen as a diplomatic triumph on the part of King Carlos I whose reign was marked by a flurry of diplomatic activity, having successively hosted State visits from Edward VII of Britain, President Émile François Loubet of France and Kaiser Wilhelm between 1903 and 1905. Acquiring even a lesser valuable portion of territory could have been perceived as a result of the King's activities and given the monarchy a repressive from republican opposition. In return for British support in this endeavour, British companies would have been granted preferential access to this region.
With rubber being the most valuable commodity at the time and most of it being present in the northern half of the territory along the Congo River, France might claim the lion's share as France was generally regarded by the other powers of the time as an excellent colonial administrator. On the other hand, it might have to make concessions to Germany in exchange for Morocco. This period coincides with the First Moroccan Crisis in 1906 and therefore ceding a large portion of the territory to Germany might be a way to satisfy Germany's colonial ambitions. The British government seemed little interested in much of the territory, but I imagine that at the very least Southern Katanga and a Northeastern region around Uganda would be annexed into the British Central African Protectorate and Uganda respectively. By obtaining the most valuable portion of Katanga Britain would have secured among the world's largest copper deposits, and by graciously allowing Germany the lion's share of the Eastern Congo, it would have perhaps put Germany closer to achieving its colonial aims. However, German ambitions will have remained unsatisfied.
