Just as an FYI, most black Rhodesians did NOT like Smith. Hell, they didn't care for white Rhodesians in general. Rhodesia was not the apartheid state, but racism was still very, very much in attendance. Mugabe's first two terms were the golden years, owing to his then-benign leadership at first, which got increasingly dictatorial over time. Mugabe ultimately vindicated the views of much of Smith's cabinet, who expected that Mugabe and Nkomo would ultimately destroy the modern state.
Anyways, the federation was going to collapse, it was simply a matter of when. A better idea would be integrating the two Rhodesias as one state, and allowing more moderate leaders (Garfield Todd, for example) to come to the front. If you stick with the leadership of Higgins and Welensky, not much can change, and adding Smith into the mix makes things worse. Nyasaland was a place where the whites were by 1960 outnumbered nearly 1000-1, and the presence of Hastings Kamuzu Banda makes keeping them part of the federation nearly impossible. Better idea might be to lose Nyasaland and reform the Federation as just Rhodesia. Keeping McMillan from making the Winds of Change speech, which sent African whites in general into a frenzy, would be a big help, too. Even then, that unitary Rhodesian state may or may not be able to hang on, considering the problems that Smith and white Rhodesians faced. I think the only way that white Rhodesia could survive is if it becomes a satellite state of South Africa. If you can keep the two Rhodesias until 1975, South Africa's war in Angola might just force Pretoria to make Salisbury a major ally.