I wonder, then, if there's a way that Kongo could go even further with its adoption of European ideas. For example, it seems that even though there were attempts to introduce European-style schools in Kongo, they were limited to clerical training.
From what I can gather from various sources, it seems that literacy was limited to the nobility, and that was usually in Portuguese, not Kikongo. There were a few catechisms printed in Kikongo but that seems to be the extent of what exists. There wasn't a Kikongo Bible until the 20th Century, and before that it appears to have been memorized. Perhaps if the Kongo nobility or the Jesuits took an earlier, more active interest in coming up with a standard written form of Kongo, the chances of Kongo survival would have been somewhat greater somehow.
So the Dutch couldn't have helped Kongo at any point? How about the English or the French later on? Since there was clearly contact between Kongo and Europe, with Kongo letters and travelers contacting the Potuguese, the Papacy, and the Dutch, I wonder why this didn't amount to further. Kongo went from being recognized as a legitimate, sovereign, Christian power to just another weak, obscure African polity to be swallowed up in the 19th century scramble. Could sending ambassadors to London, Paris, Amsterdam, and elsewhere help?
I'm imagining a more powerful, more influential Kongo that manages to obtain the entire territory of the Congo Free State instead of Belgium by the time the Europeans are drawing their maps. Since the French and the British wanted a neutral power in the area, why not an indigenous one?