To what extent - if at all - did ANC leaders oppose the necklace?
Most of the evidence I've come across suggests the opposite. Alfred Nzo (appointed Minister of Foreign Affairs in 1994 by the first ANC government) and Winnie Mandela supported it; Oliver Tambo seemed to essentially mumble something about its time having passed when asked about it in the late 1980s. ANC activists invited to US universities defended it.
Is there really any substantial evidence for the idea at all? I accept my bias against the party could be making me cynical, but it seems to me like they did support it - it was perhaps an essential part in actually making the townships ungovernable and overthrowing apartheid by including yes, scaring the daylights out of people through terror; stopping students from taking exams, stopping people from going to work, stopping people from going to white supermarkets even if that meant hardship or even starvation, so that business owners would put pressure on NP leaders to negotiate - and any distancing done before or since 1994 has really been PR work and tactical, rather than heartfelt.