Mandela pretty much had a successor in Steve Biko. There are many places in time where things could have gone differently for South Africa and the fight against apartheid, but in all honesty if it ever did devolve into a full-blown war as it did in Rhodesia it would have been brutality on a holocaust-level scale, if not bigger.
The ANC very brightly decided the better option with the armed struggle was not to attack whites, but to just hit the government. As the violence ratcheted up in the 80s the ANC did start bombing bars and nightclubs, but the violence was worst perpetrated by dumbass police forces shooting at anything that moved in the townships. If you want a full-blown civil war, that's what you do. But had Mandela been executed, there was others to take his place - Walter Sisulu, Oliver Tambo, Govan Mbeki (and eventually Thabo Mbeki), Joe Slovo (who is white, FYI), Albert Luthuli and then the second group of guys who came around in the 70s, led by Steve Biko.
By Biko's time, Afrikaners were starting to turn against apartheid in many cases too. Witness people like Andre Brink, Rian Malan, Alan Paton, Beyers Naude and Helen Suzman.
Had Biko not been so brutally murdered and instead had led the non-violent resistance, that might have been enough to stop the violence of 1980s South Africa before it happened, Mandela and the ANC around or not. Biko's death turned out to be the trigger for the roundabout circle of violence that still rages today. Apartheid was beaten but then the blacks turned on each other in big numbers, causing the crime rate that cripples South Africa today. I am of the opinion that the sanctions that damaged SA's economy in the 1980s did more harm than good, and if the population had stayed non-violent than things would have been very different. The whites by and large were not on Mandela's side in 1964 - but they were definitely starting to turn against it by Biko's time.