If you're going to pursue a case in a criminal court, you bet your ass the "old rules" still apply.
The actual “battle” so far is a PR battle. If Barr were to arrest Comey Impeachment would be a side show.
1. Trump is WINNING this PR battle.
2. I'd love to see Comey arrested. But that case has to be airtight, and understandable to the public (since this is a "PR battle," right?). The absolute worst thing that could possibly happen would be for Comey to be acquitted before Election Day 2020.
I wouldn’t care if a two year drawn out trial acquitted Comey.
Here, IMHO, is the problem in your reasoning. You think Barr would “let the chips fall” but he won’t. Once he starts arresting people you know as well as I there will be a line he will not cross. Comey, Hillary, 0bama himself. There will be a line where he absolutely will not go up the chain. Barr’s problem, again, IMHO is that he has not decided as yet just where he will draw that line. Again IMHO the delay is because he is hoping he will be “overcome by events” and he won’t have to make that decision.