The Portuguese specifically were VERY interested in West Africa as they sought the massive wealth of the Sudanic kingdoms (Ghana, Mali, Songhai). Its simply false that there was a lack of interest. Its true they didn't dedicate huge amounts of troops but they did construct several forts and one of their stated purposes was trying to tap into the immense wealth of the trans-saharan gold routes that fed Europe nearly all of its gold.
Early on that was true, but the gold trade declined at this source in the late 15th century. So much that when the dutch took Mina in 1637 it was defended by a handful of soldiers. At its peak the gold trade from the Gold Coast was about half a ton/year, declining to about 400kg/year in the first half of the 16th century and 200 kg/year in the second (Pereira, João Cordeiro, “Resgate do ouro na Costa da Mina nos reinados de D. João III e D. Sebastião”, Stvdia n.50, 1991, Lisboa). Whereas in the mid 18th century the gold trade from Brazil reached an average of 15 tons/year. Brazil became a far more attractive colony that any in Africa.
I will read those papers I can access, but the fact remains that the sole primary written sources available about that era (16th century) are the surviving portuguese administrative documents, trader/adventurer's reports, and a few from missionaries. Even Pigaffeta's
Relatione is a secondary source, reporting Lopes' stories. There simply was no one else in the region and era
both writing and archiving the documents produced. Oral histories spanning centuries are, everyone knows, entirely unreliable. Archeology is still poorly developed there and most materials were perishable, in an unforgiving climate. There are rather fierce local disputes about how old the Kingdom of Congo is, some local nationalists desperately wait it to be very ancient but the archeological record so far cannot prove that. And comparisons with other areas or later times are unreliable.
https://scholarworks.umass.edu/cgi/viewcontent.cgi?article=1329&context=adan <- A report detailing the superior qualities of Kongo-made iron when compared with cheaper work by Europeans (though yes, Europeans manufactured far more and did use it for firearms, it was just a much lower cheaper grade and wasn't crafted as well)
This seems to rely mostly on a late 17th century
secondary source and late (19th/20th century) observations of how local iron-working was done, assuming it hadn't changed. In any case it is true that ironworking was well known and practiced in many areas, and fully satisfied local needs. The only practical advantage that european iron might have would be if it was cheaper. But given transportation costs trade in raw iron so far away would not make much sense... which just led me to look at, and discover, the interesting story of an early colonial attempt at ironworks in Angola, thanks!
This seems to rely mostly on a late 17th century secondary source and late (19th/20th century) observations of how local iron-working was done, assuming it hadn't changed. In any case it is true that ironworking was well known and practiced in many areas, and fully satisfied local needs. The only practical advantage that european iron might have would be if it was cheaper. But given transportation costs trade in raw iron so far away would not make much sense... which just led me to look at, and discover, the interesting story of an early colonial attempt at ironworks in Angola, thanks!
The paper you mentioned says at one point:
The first was a Portuguese effort to establish an iron foundry in Angola in the 1750s. The foundry was unsuccessful in transferring technology to Kongo black smiths; rather, “it concentrated smiths from across the colony in one area under one wage-labor system.” Such methods were a tacit recognition of Kongo ironworking skill. The Portuguese foundry at Novas Oerias utilized European techniques was unsuccessful, never becoming competitive with Angolan smiths. The iron produced by Kongo smiths was superior to that of European imports produced under European processes.
There was no incentive to replace Kongo iron with European iron unless Kongo iron was unavailable. European iron of the period contained a high amount of sulfur and when compared to the high carbon steel produced by Kongo iron processes, was less durable, a “rotten” metal. European iron was the second choice, whether the purchaser was from Asante, Yoruba or Kongo.
The author of this paper gets his terms wrong, which is understandable as he is quoting secondary sources... but not a good sign! It was called "Real Fábrica de Ferro de Nova Oeiras", not "Novas Oerias". Nova Oeiras was near Massangano, in Cambambe, Angola.
But it seems to me that he is also wrong on his conclusions on that segment, which I can only call fanciful at best. The foundry was not unsuccessful because of any inferiority of european techniques. It was stated with local workers and local techniques, in 1766. In fact all the european blacksmiths sent there to oversee the works kept dying from local diseases before they could set up the factory. In 1767 the first 3 hired in Bahia died before even arriving. In 1768 4 new hires from Biscay survived long enough to arrive, but were all dead by January 1769! The factory was built but manufacture continued exclusively with african blacksmiths until a few prisoners from Europe were deported there (no master blacksmith would go on their own free will...) in 1770. Cynically, 30 were sent this time "because precisely almost all of them will sicked and some die, it is necessary to send enough to make up for that". But by this time the governor Francisco de Sousa Coutinho lost his optimism for development of manufactures in Africa. He was replaced in 1772 with a new governor who wrote of the project "it was a total chimera... harms the royal treasury with a huge expense and sacrifices no small number of lives in these unfruitful endeavors... I ordered all work suspended on the material of that factory." A contemporary report notes that 77 europeans and hundreds of Africas had died in the "evil climate" of the region. There are records that on the whole about 7400kg of iron was produced and exported to Lisbon, in 1767 and 1768. No further production is recorded.
(
Source: Uma Tentativa De Fomento Industrial Na Angola Setecentista: A Fábrica Do Ferro De Nova Oeiras (1766-1772), Africana Studia N.C10, 2007, Revista Internacional de Estudos Africanos, Centro de Estudos Africanos, Universidade do Porto)
Sousa Coutinho's son, Rodrigo de Sousa Coutinho, would continue what seems to have been a family inclination to estabelish ironworks on new lands

this time successfully, he founded the
Real Fábrica de Ferro São João do Ipanema in Brazil, which would start the large-scale iron industry in the country.
The purpose of the foundry works in Nova Oeiras was to produce iron for export, to supply the needs of the government in Brazil and even in Portugal. Not to replace the local production. Iron ore is widespread, refined iron for working could be sourced from relatively near, and village blacksmiths were common. Even in Europe it was common right into the 20th century to find village blacksmiths producing iron goods for the local needs: hoes, axes, etc. They eventually bought industrial iron and steel to transform (rather than refine ore) after the industrial revolution, but the final production of some goods remained decentralized for a long time even after that.
John Thornton, whom he cites, was notable for defending (he's not alone in that, imo correctly) that the african trade networks were not easily disrupted by the europeans on the coast, and that effective control of those remained with africans until the colonial era. The continued decentralized production of iron was explained not by any technical superiority but by its economic superiority.
Sousa Coutinho in fact commented on the skill of the local workers: "it pains us to see that in this centre of barbarity and ignorance the said barbarians will exceed us in knowledge because without means and without industry they will satisfy their needs and enjoy the benefits of heaven in the prodigious mines of this most precious metal [iron]". It seems to me that he did not meant that the locals were superior in technical knowledge, but that they could easily become should modern production techniques be introduced, and were already more skilled in supplying local needs. Sousa Coutinho's aim was to develop the colony of Angola beyond its role of supplier of ivory and slaves, much as his son would go on to develop independent Brazil. Hence the project of a factory that would employ only a few europeans to introduce those techniques but rely on african workers, preferably attracted through offering wages and benefits. Sadly a combination of harsh environment and officers who lacked his vision doomed this project of development, during the time when the Ancient Regime social norms could still accommodate different concepts of property and authority. The late 19th and 20th century colonization of Africa would be far less accommodating towards what existed.
There is an interesting work on the ironmaking traditions of Angola and the Nova Oeiras foundry, regrettably I can only provide a
link for it in Portuguese. It had been published as a book, there may be an english translation.
As for quality considerations, it is impossible to generalize comparisons of iron products of one continent against iron of another. Ironworking was still not much of a science and its quality depended on the specific place of manufacture. I seem to recall reading that swedish iron was dominant in the international trade in Europe, and regarded as better that english iron until the 19th century. I expect that african iron products would be even more diverse in quality.
THORNTON, JOHN, and ANDREA MOSTERMAN. "A RE-INTERPRETATION OF THE KONGO-PORTUGUESE WAR OF 1622 ACCORDING TO NEW DOCUMENTARY EVIDENCE."
The Journal of African History 51, no. 2 (2010): 235-48.
www.jstor.org/stable/40985072.
Heywood, Linda M. "Slavery and Its Transformation in the Kingdom of Kongo: 1491-1800."
The Journal of African History 50, no. 1 (2009): 1-22.
www.jstor.org/stable/40206695. <- the usefulness of this article is showing how Kongo was engaged in military expansion during the sixteenth century. For OP, Heywood wrote an excellent book about Njinga also.
I won't be able to read these anytime soon.
Thornton, John. "Cannibals, Witches, and Slave Traders in the Atlantic World." The William and Mary Quarterly 60, no. 2 (2003): 273-94. doi:10.2307/3491764. <- an account of African fears of the European cannibals.
This I could not find now but I am rather curious. Regarding the "cannibalism" reports, they need to be taken cautiously. It was a usual accusation made against inconvenient enemies to justify war, also in Brazil. Some might be true. others might not. Those in Angola against whom the accusation was most often made (the "jagas") acted as mercenaries for diverse powers, hence had no lack of enemies. All accounts about them are negative, in part due to their role as invaders and mercenary soldiers. But the term itself does not designate a specific group, it came to designate the "bloodthirsty" enemies (or unreliable allies).
The reports about fear of slavers as cannibals seem logical but I wonder at how much of it was fanciful. Where cannibalism was practiced a the region
and the slave trade towards the coast was unknown, it would make sense for captives to fear strangers with an unexplained appetite to take away people. It qualm is still in that
where: it seems to me that if (when) the slaves came mostly from the local population of Congo and other kingdoms near the coast then the slave trade would have been familiar. If they came from captives taken from among other people it could arise. Though I doubt any definitive answer can ever be given, I am inclined to believe that most of the slaves came from the coastal area which had long-time contacts with europeans and knew enough about them to see that the aim of slavery was slave labour, not eating people.
But this is kind of off-topic, it's about events after the 16th century. I will dig out the copies I have of 16th century letters and reports about the area and see if they have something useful to add, for the questions in the thread. May take a while.
Of interest to the OP may be one fact: if there is any interest for a Civ mod to represent territorial portuguese influence near the Kingdom of Congo, the period to look at is the 18th century, not the 16th. the 1760s and 1770s were the time whet a real attempt was made to occupy the interior and build a "real colony" in Angola, as was being done in Brazil. This effort faltered in face of the harsh conditions of the interior and the much more attractive alternative of Brazil, and was abandoned when Portugal was overrun by Napoleon's wars and all government support for it ended for a century or so.