why would SP Mueller need Manafort to confirm what is already known?
That's just how it works in court. The prosecutor may already know something but the evidence for it is too complicated for the average juror. It's good to present the evidence AND have a witness or a confession. That simplifies things.
Apparently, it works like this:
* Prosecutors would tell Manafort to list every crime he has committed, and then identify every crime committed by others
* Prosecutors ask about any crime that Manafort knows about.
* Prosecutors begin corroborating everything Manafort said with external information.
* When completely satisfied of all of Manafort's information, the Prosecutors can offer him a cooperation agreement.