The ANC was not even the strongest black political movement in South Africa. They became that when the Democratic congress started funding them above the other black nationalist group, the PAC, because of the liberal press deifying Mandela for 10 years before that. Mandela really had no influence on how that happened, it was the work of those who were in exile to establish him as a symbol and a martyr. They did that with the help of the American and British left.
Negotiations failed several times after he was released because he refused to disband the military wing of the ANC, and they still have not done that. In the end, De Klerk and Meyer, the government negotiators, gave in to all the of the ANC demands when Clinton gave them $2 million to "negotiate".
Source: Larry Devlin, the ex-CIA station chief the Congo