The thing about mercenaries is that they will only fight so long as they're getting paid. Nobody wants rubles, so they would expect currency conversion, and this is while the Russian economy is in the toilet. Russia also has a bad history in terms of keeping their word even if you could trust that the country weren't in a spiral, so most PMCs aren't exactly gonna get excited for any deal that doesn't include payment upfront. If Putin is jailed, I think we're more likely to see a power vacuum in Moscow than people scrambling to free him and go to war against South Africa. Putin only has power so long as he is surrounded by protectors loyal to him. I'm not sure any of them are that loyal to him at this point. He's fucked the country with this invasion, and he's getting old, and the rumor is that he's very sick and possibly terminal, but that part could be propaganda designed to get loyalists to second guess their allegiance.

Although, to be fair, Russians haven't exactly made a lot of good decisions lately, so maybe there are enough ignorant dumbasses in the region to decide to go to war. Devout nationalists might continue to fight despite empty bellies, but they won't fight well.