Okay, Nelson Mandela was huge, no doubt about that, but to suggest that without him that the nation would have convulsed into a bloody civil war is just a bit too simplistic. Besides Mandela there were also other leading moderate figures in the ANC and related liberation movements like Oliver Tambo( lived until 1993 and the co-founder of the ANC) and Steve Biko( I believe he was murdered in the 1970s however) who would have largely taken the same conciliatory approach.
The Apartheid regime mainly fell due to two factors, firstly the fact that the cold war signalled the end of Soviet/communist funding for the ANC, which managed to remove the assertion that they were socialists who would destroy the entire afrikaner capitalist edifice. Secondly, the fact that sanctions had finally started to bite, which largely meant that by the time the ANC even managed to ascend to the seat of government all their lofty social welfarist plans were dashed just due to the basic insolvency of the government.
Those two factors largely still would have been in play in the late 1980s and early 1990s, and as the ANC's militant campaigns were largely for the whole ineffective and the moderate center still somewhat in play, I predict that even with a Nelson Mandela death, we still reach, albeit a bit tricker, a ANC victory.
However if you remove the fall of the iron curtain, no way this ends in 1990 and it definitely persists all the way into the 21st century.