We also need to keep in mind that pardoning someone to keep him from testifying against you, is particularly ineffective. People who have been pardoned can't use the Fifth Amendment and can be compelled to testify against you under threat of indefinite jailing and fines. And Contempt of Court or Congress is not a pardonable offense.
Manafort is going to be a great example when he gets sentenced for the state crimes he already pleaded guilty to in his plea agreement. Since he continued to lie, the judges do not have to honor the agreement, so he can get the maximum sentences in several state prisons. Just think what that's going to do for anybody Mueller charges: They are going to foul their britches and then beg for a deal.