They can all take the 5th and this stalls until after the election....
Unlike on TV, having done thousands of interviews/interrogations, I can tell you that you collect all the pertinent documents, interview witnesses and lower level suspects, obtain confessions of these suspects and then you interview/interrogate the primary suspect. This is done in order present the suspect/target with enough of your evidence to convince him/her that it is time to make deal or risk going to trial.
If this report is accurate then Durham is at the end of the case and ready to indict the culprits.