Who thinks Zuma will go if the ANC vote is against him in December?  On past form he will stay and the masses will back him to the hilt.
Westminster or even US models do not apply -- under Zuma South African politics has become much more like the old USSR, where the chairman of the politburo held all the cards and effective power.
The most glaring example of this is his control over parliament, where the ANC vote for him as one.