Trying not to go off topic here because most of this is discussed in the Highlights thread, however things in the letters that are useful to the state are.
The third Zany story, the monkey story, she never talks about finding the real killer and that its not her battle, many of her statements such as when all else fails, deny, deny, deny...which is what she has been doing.
The letters show her frame of mind, her total disregard for her daughter as well as countless lies, and another story about what happened.
Even if Robyn is called to testify, I find it believable that KC would confide in Robyn and I think a jury would to, she confided in her about the abuse, about her family dynamics, things that upset her, as well as how she could tell her anything, so I think a jury would believe that Casey told her she used Chloroform as well as the blanket in the bag. I mean if Robyn really wanted to come up with a story she would have claimed that KC confessed to her, but she didn't.