Various witness have been “flipped” by Mueller, and have signed deals for immunity or reduced sentences in exchange for testifying for the prosecution. e.g. Rick Gates who is currently testifying against Manafort. And there is a strong possibility more will follow (coughs Michael Cohen).
My question is could those witnesses be forced to testify in a hypothetical future impeachment hearing? Presumably it would depend on the exact details of the deal reached with Mueller. Seeing as it is the stated policy of the justice dept that the sitting president can’t be indicted, it doesn’t seem too unreasonable for Mueller to consider a hypothetical future impeachment hearing during his investigation.
Even if no such clause is included in the deals struck by the current crop of cooperating witnesses, is there anything that would stop Mueller from adding them to a future one? e.g. if Michael Cohen has evidence of “high crimes and misdemeanors” committed by Trump could he be given immunity or reduced sentence on the crimes currently being investigated in NY in exchange for testifying against Trump, were impeachment proceedings be brought in the future (possibly in addition to testifying in regular trial against non-Presidents who were involved in those crimes).