Two people were given immunity in exchange for testimony specifically because they thought the testimony might give them ammunition to go after bigger fish.
Read the IG report. There was nothing odd about it and it confirms that the investigation was proper and thorough.
Mills and Samuelson had consent agreements. Not immunity.
The only reason immunity was offered was to go after higher ups. If any of these low level people had been prosecuted and not the higher ups, they’d be complaining that they found scapegoats. The only outcome some people would accept is prosecution of Clinton despite all evidence against it.