But, IMO, even if Robyn was using this as a bargaining chip, she will only be discredited, not impeached and the jury can still weigh her testimony. Coupled with these letters, I think that testimony will hold weight.
You are right that the jury could still weigh her testimony. Would it even get that far though? This is a criminal who has no proof to back up those specific claims and who also clearly states that she has struck up a relationship with Casey to get Casey to talk about her crimes so then she can use it to get money or to get her prison sentence reduced. Would the prosecution risk allowing that hearsay on the stand given that they seem to already have very good evidence elsewhere, that they can back up?