Regarding Barr, there will be a point in the inquiry that will be recognized as legitimate in the same sense it was in US vs. Nixon.
At that point Barr cannot hide behind Executive Privilege nor any legal excuse as this has already been decided. Assuming that Barr is summoned, he either appears or he is held in contempt of Congress, the courts or both and is subject to arrest and prosecution with the likelihood that any pardon in self-interest will be taken up by a rather unhappy SCOTUS, assuming Barr is stupid as Cohen and others believing that he has a guaranteed pardon.
It's hard to see how Barr isn't in the hot seat in Congress and court with criminal consequences being a very possible outcome. At that point, Barr is open to investigation outside of his connections with Trump proper in a "follow the money" investigation.
Few things could please me more.