To the first question, whether she was an actual person, there's no doubt of any kind. Although there's a lot of mythology surrounding her and that iteration of Nyabingi/Nyabinghi as a mansion of Rastafari, as a person and a leader of a devoted following, there is no doubt. Official records and period accounts attest to her troubling the Germans, although I'm trying to find an account of her internment (really a form of imprisonment) by the British in Kampala. I know there's at least a few despatches in CO 536/64 (British National Archives, Kew)--I have the index around here--because this was a Foreign Office issue involving the Germans, and I'll have a look at my Africa: Confidential Print files to add more here if I can.

One accessible source that's not behind a paywall is M. J. Bessell's account form The Uganda Journal in 1938. It's a hybrid account, and full of period presumptions about motivations and activities from a mission-colored point of view. Still, the chronology is fairly good relative to Uganda. A lot of the general material available is in German or French [edit: and still in Uganda or Rwanda], as one might expect, and as Bessell's source note indicates. Most colonialist accounts impute a strongly millennarian and anti-colonial element to the spirit possessions and the risings that punctuated the era, which is not the whole story, but would be in keeping with the experience elsewhere in East Africa (e.g., Maji Maji in southern Tanganyika).

The second question is harder to get at through the mist of mythos. Just as the mission-influenced Bessell has a vested interest in downplaying her role and spirituality, others have an interest in playing it up. What we can say is that she rejected the pro-German claimant to the chiefly throne she oversaw, and rose against the Germans in Ruanda-Urundi (at the time). This did not work out so well, and she was detained by the Germans first (at least twice) and the British later in the Uganda Protectorate until her death in 1945. Complicating any search for period accounts of her is the multiplicity of names and spellings used, and of course the many agendas at work that time and since.

If you're willing to do the library crawl, there are unsurprisingly better accounts than Bessell's out there for a general reader. The most recent treatment I can think of dealing with the entire movement is chapter 3 ("Religious Movements in Southern Uganda") in Derek Peterson's Ethnic Patriotism and the East African Revival: A History of Dissent c.1935-1972 (Cambridge: Cambridge University Press, 2012). He provides a LOT of footnotes to work that goes back quite some way, and talks about the early era despite the "c.1935" in the title. Murindwa Rutanga's essay "People's Anti-Colonial Struggles in Kigezi Under the Nyabingi Movement 1910-1930," in the special issue of the Viennese-published Journal für Entwicklungspolitik edited by Mamdani and Oloka-Onyango (Uganda: Studies in Living Conditions, Population Movements, and Constitutionalism) in 1994 is also one you probably want to chase down. I had a copy back when I was working up seminar studies regarding millennarian challenges to colonial power, but I never pursued the case of Muhumusa/za and Nyabingi/ghi because so much relevant material was not accessible to me compared to, say, Kinjinkitile and the Maji Maji rising. Older works from the 1950s-70s that Peterson and Rutanga cite are useful in part because some of those scholars actually interviewed people who took part in that movement or knew the key figures personally. [It's worth adding here that some of those studies were by African speakers of Kinyarwanda or other close regional dialects, which gives them a great deal more interpretative usefulness than a lot of the same era's works by people who were culturally or linguistically foreign from the region in origin.]