Under this scenario the marches will only "explode" in any way meaningful to Zuma if, as well as Cosatu, ANC members and branches take part too.  Which is not likely to happen as they are thoroughly corrupt.
The army are less of a factor than the police, and all the signs are that the police are absolutely pro Zuma, ready to lie and block in in the courts in any political cases.  Enthusiasm for marches fades among South Africans when the bullets fly and converts to low-level political gangster and war lords in the townships, something the security apparatus is geared for.
His main problem is getting around the two term limit in the constitution.  My money is on either "national emergency" scenarios being used (invasion of Zimbabwe, Botswana or Moçambique, nuclear threats from Trump, IS planning a takeover -- the choice is large and he can take his pick) to justify a sine die extension, or the constitution being overturned through MPs being offered big bribes.