IMO, the idea is they will not use what she says from this point forward to prosecute.personally, I agree with you. But if LE believed that, they would not offer immunity. Show me one homicide case where they offered immunity, of any kind, to a person whom the prosecution considered SOLEY responsible. Yes, it's a challenge.
But they won't have to use it. There is enough evidence, if this test is conclusive.
So perhaps, they know they can convict without her cooperation and that is why they are being generous, but they just want answers.
just a thought.
Mark Hacking acted alone and I THINK he may have gotten a concession in his plea deal. not sure..can't recall.