Not quite. The Europeans only really started taking an interest in the African slave trade after the Portuguese got them interested, and demonstrated the potential rewards which could be generated from it. The examples which you cite are examples from well after the demand was already established, from the 16th, 17th & 18th centuries. And those wars weren't fought with the purpose of capturing and selling slaves- they were financially supported by capturing and selling enemy POWs as slaves, with the capital generated from these sales helping to stave off those states' bankruptcy, and to sustain their war coffers over the course of campaigns which often endured for decades.
And even IOTL, the slave trade went both ways- for most of the middle ages, up until the 16th century (after the coastal African slave trading ports became well-established), Europe was actually a net exporter of slaves to Africa (enslaved and sold into African slavery via the Arabs and the Trans-Saharan slave trade, in order to support Europe's own crippling endemic warfare), not the other way round. There's a reason why slaves are known as 'Slaves', and not 'Abds'- for the Arab slave traders, Europe was the place to go and buy slaves from, not Africa.
In an ATL where the military technology of Africa more closely parallels that of Europe, as a result of gunpowder warfare being introduced on both continents at roughly the same time, it's by no means guaranteed that the expansion of the slave trade will result in Africa becoming a net exporter of slaves to Europe ITTL. With gunpowder warfare, conflicts in Europe became far more intensive on human resources; war campaigns had to be conducted far more quickly, larger armies would have be mobilised, and those armies had be far better equipped and supplied (incurring far greater expenses due to the more difficult logistics).
And as such, Europe was left facing a labour shortage- one which it met by expanding the institution of slavery, and exponentially increasing its importation of non-European (African) slaves to compensate for its inadequate workforce. Endemic gunpowder warfare effectively necessitated Late Medieval Europe's change from a net exporter into a net importer of slaves IOTL, enabling the European kingdoms involved in the slave trade (whose own populations had utterly stagnated) to keep their economies viable and maintain a viable workforce through the importation of slave labour.
ITTL, the Africans would be in exactly the same boat, facing the same challenges. Soon enough, the Africans would be looking to increase the importation of increasingly valuable slave labourers from Europe to boost their own war-ravaged workforces ITTL. And plenty of players in the European slave trade- the Berbers, Arabs, Radhanite Jews, Venetians, Crimeans, Knights of Malta and even the Vikings' successor states- could well inclined to expand their own slave trading operations in order to cash in on the far higher rewards which they can reap by exporting European (and Islamic, and other Asian) slaves on to African Kingdoms. IMHO, it'd be far more likely that the Arabs and other intermediaries would become the 'parasitic wasps'; the European economy may well be just as vulnerable to re-routing ITTL as the Asian and African economies of TTL are.
As for the other bit- IMHO, if TTL's Africa ended as a patchwork quilt of Japan and India analogues, I'd be inclined to deem that as a scenario in which Africa would be sufficiently 'saved' to be counted as a huge Africa-wank TL. It'd certainly be far more of a win for Africa than OTL. And, of course, there'd be no telling what the rest of the world might look like as a result...
I know about the white slavery and whatnot into North Africa, yes. and about the etymology But sub-Saharan Africa was not receiving slaves--it was exporting them north to North Africa or to India/Iran/Mesopotamia thanks to the Arabs.
And white slavery in Europe proper ended in the 1000s. The Church did not allow it, the Radhanites collapsed, the Vikings converted, the Venetians largely traded pagans to Muslims, and the Knights of Malta never engaged in slavery. By the time you get to the 14th century, the major slavers are going to be Muslims raiding Christian areas, and said Muslims were bitterly opposed by Europe. Christians were not going to sell other Christians into slavery in Europe proper; the reason they got away with it in the New World is because racism was developed from Arab sources and Hamitic theory to justify the keeping of African slaves.
I never addressed the initial premise; I don't think gunpowder was going to get into the Sahel that early. It came from China along the Mongol trade routes--to the Middle East, Russia, and then Europe. It then went into India, the rest of Asia, and North Africa--I don't see gunpowder getting that far past, at best, the Swahili Coast or Ethiopia. It took a while to transmit for a reason.
Point by point: Those wars were definitely fought to get PoWs to sell as slaves. You had non-state peoples raiding Kongo and Lunda in Central Africa, you had the various Ashanti conflicts with their neighbors, you had the Wolof and Fulani fighting... anywhere from Senegal to Benguela had some amount of slave trafficking going on. Other major states, like Zimbabwe, collapsed and then reconstituted itself as Mutapa. Mutapa was divided between Portuguese and Arab-Swahili influence; the Arabs largely won out, because Portugal didn't have the men to go an fully conquer Mutapa. Eventually, their backwoods settlers were kind of wiped out by the replacement of the Mutapa with the Rozwi and others.
Even with gunpowder in the Sahel, the existence of a lucrative, constant-demand trade market will lead African states to respond to that demand by selling slaves. Even the Asian states sold slaves to Europeans, although eventually both the Tokugawa and Chinese cracked down on sales. Since Europe had the navies, they were going to be the ones discovering the New World, leading to the death by disease-and-slavery of the natives, necessitating the need to import tropics-proof labor to start up the booming sugar industry.
Europe also had a number of other advantages--a tangled but sophisticated legal and administrative "layering" system, large mercantile classes that had immediate coastal access, etc. I don't see Kanem and Mali a) getting gunpowder that early or b) being able to coalesce from feudalism to modernity so quickly. You mention the Black Death--the labor changes engendered by the plague, along with the rest of the crisis of the 14th century, allowed Europe to trade feudalism for the beginnings of the modern state. Africa, as mentioned, never suffered said plague, and thusly had much less of an impetus to transition out of feudal/pre-feudal organization.
There is also the Arabs. Although I highly doubt that they'd be able to do to Europe what Europe did to Africa (the European mercantile classes are too strong, already had a large role in trade with Muslims and were themselves the intermediaries, and then you have the end of the Reconquista etc), they were able to dominate the major candidates for advancement. It only takes one Moroccan invasion of Songhai to destabilize the Western Sahel for generations as the successor states squabble amongst each other; the Swahili were already fractious and eventually fell under the Omani aegis. States like Kongo fell under quick European domination and European-started instability, and in any case I don't see how the Lunda or Kongo would be getting gunpowder contemporaneously with Europe.
