I wouldn't be surprised if she's alive.

However, he balefired the fortress and the man was suddenly fine. That is pure balefire text if ever I saw it. Rand's also not ignorant anymore - he and Lews Therin know how Graendal works, and how Compulsion works in great detail.

My biggest problem with your theory is that it would require two things...
- perfect timing : her weaves would have to unravel at the moment of his attack, and there's no way she could know this. He could have attacked her minutes later or hours later - she could never know.
- the absence of residues : the weaves were completely gone. They didn't unravel and leave a trace. Nynaeve would have noticed this.

Graendal always insinuated that if she detected Rand she would flee, and I wouldn't be surprised if she became suspicious, Compelled the guy and Travelled.

I wonder if it's possible that she was not even present. Could anyone else duplicate her feat on this messenger, thus also setting us up for her return? It might be as simple as Graendal dying in this book, but I think it's possible she's alive in some twisted form. The absence of a body is always dodgy, and we didn't even see her at all in this scene.