The case against Kara may be weak, but it would surprise me if the prosecutors feel they need her testimony in any way.
They have plenty of documentary evidence of her testimony (all her email, blogs etc.) that could be admitted in some circumstances.
But more so, unless they have a problem with admissibility, they have a rock solid confession from Ludwig, found the murder weapon where he said it was (under the front seat of his car), etc.
They just don't need her testimony as far as I can see, so they have no incentive to deal for it.
If they made the decision not to charge her, I doubt it had anything to do with wanting to deal.
We have had a couple of folks walk because such evidence was excluded in the absence of the author's testimony.
But of course I don't know the rules of evidence in PA criminal prosecutions. I'm just guessing from my own experience in a different state (and one that has some wacky evidentiary rules, at that. . . at least until this year, when the Evidence code was extensively overhauled. And we'll have 3-5 years of utter confusion while the courts try to figure out exactly what it was the boys in the General Assembly meant to do . . . )