It is my opinion that there has been further correspondence from Abby (or at least appearing to be from Abby) other than the one letter that was made public knowledge.
Yet I also believe that Zenya and others do not believe that Abby is gone willingly. This could mean several things, however-- believing that a fifteen-year-old doesn't have the wisdom to willingly choose the be gone, that she is being taken advantage of, has been made mentally vulnerable, and/or is being physically kept away by force.
This is mostly just speculation and opinion on my part. But it is the impression I have gotten from comments by Zenya and WS posters who are close to Zenya.
To me, it doesn't make a difference. Abby is barely fifteen and needs to be home. At fifteen there is no good reason to be gone from home so long-- you lack perspective, life experience, and are too easily taken advantage of. Not to mention at fifteen-years-old I feel that Abby must be with someone older-- and IMO there is no way that person is well-meaning. IMO, even if Abby originally thought that she was leaving voluntarily- any attempt to return home would be met by force. It is not a normal, healthy adult that takes away or harbors a fifteen-year-old from her family.