This is a particularly egregious case (I can't see why criminal charges haven't been brought against Fina for civil rights violations), but prosecutorial overreach is pretty endemic. I think the lesson for everybody should be -- DO NOT cooperate with prosecutors unless you're given a comprehensive immunity. Statements from prosecutors that you're not a target should NOT be believed.
If you do get subpoenaed to testify to a grand jury, either plead the 5th (the Supreme Court has given very
wide latitude to people who do this), or sit down with an experienced criminal attorney and hammer out a careful one page summation of what you are going to say, and plead the fifth to literally everything that is not on that page.
Curley, Shultz, and Spanier were all trying to be good citizens, just like a lot of naive people with no experience with the criminal justice system.
They thought they were helping prosecutors investigate the Sandusky case -- but in fact Fina had a secret agenda (Corbett's agenda) to bring down Spanier and they all had their careers and lives ruined as a result. If they had pleaded the fifth they would have still been legally liable based on documents and testimony of others, but they wouldn't have been charged based on their own self-incriminating grand jury testimony.
Prosecutors are great, we need them to clean up our world and put bad people away. But they are like fragmentation bombs. You get anywhere near them, you are liable to be hit by the legal shrapnel. It is very rarely in your interest to cooperate unless they really need you and you and a lawyer have hammered out a written deal that protects you.