Manafort’s doings were very well known for very many years. Don’t kid yourself.
Maybe so. I'm guessing. But, I would expect Trump's intelligence on swamp operatives to not have been complete early on. Maybe he should have known or Manafort's skeletons, or maybe a trusted adviser (Kushner at the time?) should have known enough to tell him.
Trump did fire him after only a month, likely because some of Manfort's problems were coming to light.