I am not so sure of that. I am not a law type, but conspiracy seems to apply differently. For example, collusion is not a crime, but it seem to me that it was said that Mueller hoped to prove conspiracy to collude against Trump. That never really developed, so I really am not sure.
I have heard it said that conspiracy is difficult to prove, so if that is the crime that AG Barr has in mind it would make sense to get the perps to turn evidence against each other. In order to accomplish that, individual crimes must be used to threaten individual targets, so perhaps that is the ploy of 'decline to prosecute'... I am really not sure about that - once the govt 'declines to prosecute', does it still have leverage?
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I think I am a bit too tired prepping this morning for this hurricane. I sure hope it does turn north tomorrow - maybe at least 100 miles off shore east.
Collusion and conspiracy are synonymous. One can collude to commit a crime, or conspire to commit a crime, same thing. Conspire to conspire is nonsense.
What Mueller was charged with finding was evidence that the Trump campaign conspired with or worked with Russians to break election law - could be as little as taking a $5 donation or coordinating a $10 ad on facebook.
DOJ declination to prosecute Comey for leaking his notes is the right decision. Most leaks of classified material are not crimes. Criminal leaking involves defense information, cryptography, or identity of covert spies in foreign countries. Not to defend leaking, just to emphasize that criminal leaking of classified information is a tiny subset of leaking classified information.
To answer your question about the ongoing effect of "decline to prosecute," it has no legal effect. A prosecutor has until the statute of limitations runs to change his mind. As a practical matter, decline to prosecute is (maybe one or two exceptions out of tens of thousands) always a final decision. Looked at it, made a decision, moved on to more important things.
Prayers that you and those around you make it through the hurricane with no significant harm. Somebody is going to get it for sure, this is a big storm.