If the A's have done nothing, then they don't have to worry about being charged and will not need immunity. Not understanding the problem here..
Them being charged doesn't have anything to do with wither the LE has enough evidence to for court. IT has to do with wither or not the A's are guilty of something worth being charged for. That is the whole question.
IF they have done something worth being charged for, then they might want to plea bargen to save their butt. That is what those drug dealers, etc. do. They roll over and give evidence to convict people that would 'get away' without their help. LE need their help. In this case, LE don't need their help. They don't do it for a good 'story.'
You have said that you don't believe they have done anything worth being charged. That is what I would figure as well. What throws me off, is that they believe they have. And their atty is talking about this now. That sounds rather big woops!