Sessions should fire Mueller and then resign.
I don’t care what Manafort has been doing or not doing. Nobody elected him to anything.
But Mueller doesn’t have to prove anything about him. He can use the corrupt and unconstitutional power he was given to financial ruin Manafort until he agrees to say anything Mueller wants him to about Trump. That is Mueller’s game - an economic rack. HE is an evil, unprincipled S.O.B. You can judge a tree by its fruits and Comey is a protégé of Mueller’s.
For Mueller, the worst thing that can happen is for a target of his investigation to fire his lawyers. At that point, the guy has nothing left to lose. At best, the prosecutor has no leverage over him. At worst, the guy has no leverage over himself, either.