Everything that you say makes perfect sense, except there are two qualifying factors:
A witness subpoenaed in another state cannot be extradited in a criminal proceeding. KK simply had to "take the fifth" and refuse to appear. This is not true of evidence, and it's not true in a civil suit. However, third party witnesses are not always hearsay in a criminal trial for Solicitation, so the RR's and the elusive BFF are both subject to testimony, and cannot "take the fifth".
In a "no body" murder trial, there is an extensive opening phase in which the prosecution attempts to establish proof that she is no longer living the life that is expected of her, and nor is she in the places that she would be expected to be. To do this, hundreds of witnesses can be called by the prosecution, and each one is subject to cross examination. Likewise, the defense can call anyone that ever knew KB. Any of these witnesses can later be called back, if there is anything in the testimony that would have bearing on the actual charges against PF.
I don't think she needs to be extradited; I think she needs to be transported. This person is not going to take the fifth, that happens in movies. She can be charged or she can testify. Having serve federal subpoenas (which we don't know if this case against KL is) she can be extradited. This happens every day. I think is a victim. I think she was solicited violating federal and state law and the phone transport was a federal offense. I think she talked from day one just like she did to her friend. I think clear and convincing evidence will be presented of threat to her children because he has such great character and that will take care of the doubts of her involvement. Don't get me wrong I think they are both guilty and should spend life in prison if the evidence presents that way.