IMO, Hope was behind those letters.
The detail about Haleigh's clothing sounds like someone who knew her. The person describes Haleigh's clothing including Haleigh's cute little hat. I have never heard mention of Haleigh and hats.
Hope is also a person whose life changed that night. Hope, who idolizes her cousin Ron, who thought she was going to get a slap on the hand, who can relate to Misty's personal experiences in a way that some may think Misty was writing the letters. Hope is the perfect person to "pretend" to be Misty. Pretend to be Misty to protect Ronald by destroying Misty's credibilty BEFORE she goes to court.
The one thing that made me think it was Hope was the part that said:
"Everyone sees me smiling when my heart is filled with tears."
The only ones who smiled in court were the Cummings. Misty, Tommy and Donna NEVER smiled in court. They were not defiant. They were scared. But Ronald and Hope always treated court like a joke. Well, Hope did UNTIL she was sentenced. Then the smile disappeard and the tears came.
IMO, that is where she messed up. Hope would have no idea that unlike her, Misty never smiled in court. Hope thought Misty would feel like she did. However, Hope forgot that the Croslins' experiences in Court were different than the Cummins. The Croslins would never smile in court because they never got off. The Cummings smile in Court because until this time, they always got off.