Maybe if a different disease struck Europe (instead of the Black Death), it could also reach Africa, and wipe out most of the native tribes, right before the Europeans explore it.
The Europeans will still need to restore their own population losses before colonising in large numbers, and I don't see how the African populations are at a disadvantage in this regard. Unless some freak accident of genetics creates a plague that is bad for Europeans, but absolutely hellish for Africans, the recovery curve will be broadly the same, and in terms of exposure and acquired resistance/immunity, Africa is actually at an advantage over Europe (except for respiratory infections which usually only kill in true virgin-soil scenarios).
As the African social system based on slavery is collapsed through the plague, somehow Portugese merchants take advantage, discover diamonds; and use them to fund wars in Europe, like OTL Spain did with gold from America.
Whoa, oversimplifying much! Some African polities would collapse, not least because (being essentially based on personal ties) they collapse more easily than territorial states of the Eurasian model. But especially 'tribal' rural societies are very resilient in the face of disaster. You won't see the collapse of society across the continent just because a large number of people died from disease - certainly not in the areas where megafauna is still present and huntable, but likely not even in areas where life depends on agriculture only.
Diamonds, I think, are problematic for several reasons, though they would be interesting. First, I can't recall any sub-Saharan African civilisation that actually used diamonds, which means locals aren't likely to show the conquerors where to find them. The deposits themselves are usually not conveniently coastal, so finding them would require either a systematic search (as per the nineteenth century) or a really lucky break. Secondly, diamonds aren't money. If diamonds - then a moderately popular, extremely rare gemstone - enter the European market in large numbers, they are liable to quickly saturate it. The Portuguese would probably have a good incventicve to sell them on the more receptive and richer South and East Asian markets, but it's still not the same as a money mine. Gold can be freely exchanged for anything, and an abundance affects the entire money market by creating inflation. Diamonds have to find a buyer, and abundance will only affect a small part of the gemstone market, creating glut.
Will other countries try to go to Africa, or will they head to America?
Given most European countries OTL tried both, I can't see why not. Of courese the American 'second best' bits might in the long run turn out the bigger prize, much like the British were disappointed the Dutch only left them bases on the boring Indian subcontinent and kept all of rich, glorious Indonesia for themselves.
