Another option is Issa is playing a bluff. He may not have that one piece of damning evidence he needs but he is making Holder think he does.
I was thinking more or less along the same lines. Like a dollar bill on a string or something. See what turns up.
Maybe it isn’t about Holder? In your dollar bill analogy, maybe he is dragging some bait through the water to pick up some smaller ‘fish’ who will take it out of fear? Holder had to give instructions to others to take action. Maybe the others think, if Holder takes the fifth, they will be next to be looked at so Issa is trying to get them to flip against Holder first?