"I think Mueller doesn't have anything on collusion," said Former Prosecutor 2. "I think we would have seen it. I don't see anything that looks like there's a crime lurking — maybe he's got eight indictments under seal, but to me, it makes no sense. All of this says to me there is no there there."
You can't completely discount the possibility the prosecution is taking the indirect approach on a couple cases. Manafort has the most direct tie to the campaign, given he was the manager and offered briefings to Deripaska through Kilimnik. Cohen's involvement with the payoffs is campaign related, but outside the SC scope. Butina is on the margins.
The direct evidence of collusion may be difficult to make stick as an indictment, on its own. So, you use the easier charges to prove against Manafort, Cohen, or Butina as leverage. Their cooperation would be able to possibly corroborate other information that would make indictments more airtight. Manafort seems to face the most legal jeopardy, but Cohen likely has the most damaging material.