The discussion of possible Ottoman consequences is fascinating. It's also very helpful since, frankly, I haven't yet thought through all the long-term implications outside West Africa. Allow me to make a few observations:
1. West Africa is a peripheral part of the Islamic world, and religious movements that start there don't often expand outside Africa. There were a number of interesting movements in the 19th-century Sahel, many of them Sufi-based - the Mourides of Senegal, for instance, are pretty awesome - but there aren't many Mourides outside Senegal and the Senegalese diaspora. Part of the problem is geographic isolation, part is that West African Muslims follow a different madhab (most of them are of the Maliki school, which isn't widely followed elsewhere), and part is the way many Arabs and Ottoman Turks of the time looked upon Africans. This is why I envision the African diaspora, rather than the center of the Islamic world, as the primary vector for expansion of Malê theology.
2. With that said, I do intend "Belloism" (which I'll have to find a name for) to have a wider influence throughout Islam than the Malê philosophy. As you point out, it meshes well with Ottoman secularism. Also, while individual Malê will make the hajj and discuss their beliefs with fellow pilgrims, Bello will actually live and preach in Mecca for more than a decade, and will have a better chance to spread his doctrines to influential people throughout the Islamic world. There will be a secondary wave of interest in Malê doctrines as a tool of resistance to colonialism and other forms of oppression, but this will mostly take place in the twentieth century.
3. As to Ottoman ambitions in Africa, I suspect that Egypt would be a major obstacle. At the time of the POD, Egypt already had de facto independence under Mehmet Ali, with the British as guarantors of his dynasty. After that, the Sublime Porte was only able to interfere in Egyptian affairs when the great powers (particularly Britain) wanted them to do so. The Suez Canal will still happen in this timeline, and the British will, as in OTL, protect their interest by maintaining an ongoing presence in Egypt. I don't quite see even a resurgent Ottoman state being able to project much power in Africa with a British client kingdom standing in the way. Also, the Mehmet Ali dynasty tried to expand southward and westward, annexing Darfur and invading Ethiopia, but were halted by the Ethiopians and found Darfur very hard to hold. Again, I don't quite see them being able to stop the colonial powers from doing whatever they're interested in doing in West Africa.
4. The Russo-Turkish War does seem fairly easy to avoid - I suspect many of the prelude events, such as the independence of the Balkan states, are unavoidable, but even a slightly more conciliatory policy in either St. Petersburg or Constantinople could have forestalled an outright great-power war. Even a modest Belloist boost to reformism in the Ottoman Empire could make a difference. I guess I'll decide when I get there.
5. Revolutions in the Arab world: I think 1848 is too early, for reasons which have been explored in your discussion. Later on, though, we may see more Muslim involvement in revolutionary movements, which will not be so Christian-dominated as in OTL. Egypt may be a place to watch.
6. Britain vs. France in Africa: France will definitely be checkmated in the eastern Sahel, and I doubt they'd be able to establish a presence in the areas that will become Chad and Niger in OTL. They'll also be kept out of Benin. The western Sahel, however, may be another story: the French already had a longstanding presence in Senegal, and their expansion into the interior and along the coast began fairly soon after the POD. A good deal will depend upon what Louis Faidherbe does in the alternate timeline, and also on the extent to which the British are willing to (or are forced to) parcel western Africa into spheres of influence. As a spoiler, I'm planning for there to be a Fashoda analogue, in West Africa rather than Sudan, which involves the Malê, but while France will be checked, it won't be counted out entirely.
7. The absence of ethnic nationalism in Malê theology - which you correctly point out - is inherent in the nature of the Malê themselves. They're a mixed people, descended from nearly all the Muslim ethnic groups which were taken to Brazil as slaves - the largest component is Yoruba, but there are also Hausa, Fulani, Mande and even Wolof. Most of those whose families have been slaves for more than one generation have mixed ancestry. Moreover, because the Malê who were deported to Brazil were a military group, men greatly outnumbered women, and most of them had to marry into the conquered populations. This prevented them from becoming a distinct ethnic group and developing their own nationalism (although there will be Brazilian cultural survivals and they'll develop at least some ethnic characteristics). Their theology will thus not be confined to or tailored for any one nation.
8. Finally, a note about presentation. Right now, I'm working with overview posts that cover two or three years of action, with "story" posts at about the same interval, sprinkled with an occasional literary or cultural sidenote. I have a series of 11 to 13 posts planned (depending on how many sidenotes I end up including), which follow this pattern, that will take the timeline through about 1854. After that, though, I'm planning to speed things up somewhat, at first to five-year intervals and eventually ten- and twenty-year intervals. I'm thinking that this will be more appropriate as macro-effects begin to take precedence, but at this point nothing's etched in stone, and any thoughts would be welcome.