I just had to write a comment on your story because my daughter has lived all her adult life in various African countries, and the colorful details of your story remind me of what I have heard from her. The question of "self-injection" has been debated in this forum, pro and con, but there is no doubt in my mind that some of our richest and most realistic tales result from our mining our own experiences.
Given that the pronouncing and receiving of curses is an active aspect of many cultures in Africa and elsewhere (my daughter had an interesting experience in Cote d'Ivoire), it doesn't take much tweaking of the story to make it fit into the Potter universe. The magical bureaucracy in Ghana can be a reflection of the secular bureaucracy in Ghana.
In your story the curse was genuine and the curse-breaking ritual had a genuine effect, but in real life the lines between real and fake magic are blurry. Do the curses and cures actually work, in real life, simply because the victim believes so strongly in them? You answer this question within the realm of your story by having them work at a distance; Sarah Bones suffers the effects of the curse without knowing about the curse and gains the benefit of the cure without knowing about the curse-lifting ritual.
And I loved your little detail about the pathogenic organisms in the water becoming spell-resistant. That's such a clever take-off of the problem of pathogens becoming resistant to the drugs we use against them.
What worked least well for me in your story was the boyfriend-girlfriend complication between Susan and Kofi in the time leading up to the curse-lifting ritual. I can understand injecting some tension right before the climax by introducing some element that threatened to derail the ceremony (and Sarah's cure) at the last minute, but I would have chosen something other than a romantic entanglement. I see their relationship as strictly business, a friend helping a friend; if it was supposed to be something more, that could have been developed more, though for the purpose of the story it did not need to be anything more.
All in all, an interesting and enjoyable story, full of rich cultural detail. It brought back memories of my visit with my daughter when she was in Madagascar.