Willis would have had to put on a trial charging conspiracy, where none of the co-conspirators would have been present, and prosecuting would have been a very difficult process, and would have shown the strategy for future prosecutions. Powell was facing seven felony counts, so dropping those, and avoiding a costly trial helps her, of course.
As for Powell as a witness, I'm not sure how that would go in a conspiracy trial. She's not going to be a reliable supporter of the conspiracy theory, and she knows her way around a courtroom, and the law.
This to me looks like a clean-up by Willis, seeing that she has far too much to handle, and is trying to get rid of some of the mess she knows she created.
There’s a lot of defendants in this one. I think Powell’s testimony will implicate Cathy Latham.
Will Latham talk? If she does, will she implicate someone higher up?
And so on?