I feel like any area that is already so unstable that a handful of ex-Marines have a real shot at knocking it over is probably about to fall over on its own regardless, so why would they bother hurrying the inevitable?

That's pretty unrealistic. Your average ex-sergeant-current-mercenary isn't fiddling around in Congolese politics trying to drum up trouble.

why go through all the hassle of destabilising when it'll do that by itself, they are just there because that's where the steady work comes from.