Comey is compromised because his boss is compromised. It’s so obvious. Let a grand jury decide. That’s the only honest course of action.
There are two (or three, depending on how you count the prosecutor) check points.
The first check point is the investigator. The threshold for taking action by this entity is "probable cause." This threshold is used to justify arrest, and to bother the prosecutor. Arrest is discretionary, but the FBI has that power.
If there is no probable cause, the case stops right there. It never gets past the investigator.
The next check point is the prosecutor. When the investigator finds probable cause, the case is turned over to the prosecutor. The prosecutor can decide to engage a grand jury, or not, for various and sundry reasons.
Comey can kill this with a fining of "no probable cause a crime has been committed." The prosecutors are totally out of that picture.
His only other option is to find probable cause a crime was committed, at which point the prosecutors are on the hot seat.