Manafort joined the Trump campaign when basically nobody with political experience would. It was the era of the Never Trump and Cruz campaign blacklists. I doubt he was planted. Occam’s razor fits here, imo.
I'm damn near certain that he was, as it would make perfect sense considering all else that we know happened involving the Dems colluding with Russia while, at the same time, planting evidence to make it APPEAR as if Trump was.
Manafort was deeply involved with pro-Russia people in the Ukraine. Very high-ranking people in the KGB/FSB Putin administration were reportedly working to hurt Trump and help Hillary.
It should come as no surprise to anyone, at least not here, that the Russians would have waay more wanted Hillary over Trump, given all that the Obama-Biden-Hillary admin did to strengthen their hand militarily and otherwise. This, as team O-H practically, and no doubt purposely, crippled OUR OWN defenses.
So of course the Russians wanted Hillary to win.