I think you may be onto something here.
However, I think we could have a scenario where in the fusion of 1933, when the National Party and the South African Party united to form the United Party, we would have two breakaways (I think in this TL, even with Rhodesia added, fusion is still likely).
We would thus have the United Party, the breakaway HNP, and then perhaps a reconstituted Dominion Party, made up of MPs who have no desire to be in the same party as Afrikaner nationalists. There would probably be a couple of Rhodeisan and Nat MPs there.
So, come the 1950s we have:
The Nationalists - the party for Afrikaners and apartheid, draws support from rural Afrikaners, but also support from urban Afrikaners and intellectuals
The United Party - a real 'national' party, drawing support from urban whites (English and Afrikaans), with significant coloured support.
The Dominion Party - supports racial segregation but continued close ties to Britain. Support mainly in Natal and Rhodesia, with pockets in the Transvaal and the Cape.
And then, as you said, a Progressive analogue, urban liberals, calling for a qualified franchise.