I think you need to explain that, because a prosecutor cannot effectively use the lawyer who Trump hired to deal with the situations as a witness against Trump if the lawyer has not plead to the crime himself.
I don't think any prosecutor can effectively use Mr. Cohen as a witness other than corroborating voices on tapes and that the documents are true and correct copies of the documents he had in his office. And I'm not sure he is even needed for those things. He is a Trump lawyer, he is not credible.